
n^ TJ4-7 8 

Book. ,LS5 

CcpigtoF. 



COPYRIGHT DEPOSIT 



STEAM ENGINE INDICATORS 
AND VALVE GEARS 



A PRACTICAL PRESENTATION OF MODERN TESTING 

APPLIANCES AND METHODS USED TO PRODUCE 

MAXIMUM EFFICIENCY AS APPLIED 

TO THE STEAM ENGINE 



LLEWELLYN V. LUDY, M.E. 

// 

HEAD, SCHOOL OF MECHANICAL ENGINEERING AND 

PROFESSOR OF EXPERIMENTAL ENGINEERING, PURDUE UNIVERSITY 

AMERICAN SOCIETY OF MECHANICAL ENGINEERS 



ILLUSTRATED 



AMERICAN TECHNICAL SOCIETY 
CHICAGO 

1918 



S .v»* 



COPYRIGHT, 1912, 1913, 1918, BY 

AMERICAN TECHNICAL SOCIETY 



COPYRIGHTED IN GREAT BRITAIN- 
ALL RIGHTS RESERVED 



JUL 20 1918 



©CU0O97OO 



INTRODUCTION 

JAMES WATT was responsible for many important develop- 
ments in connection with the steam engine and one of these was 
the "Indicator Diagram". By means of this ingenious graph of 
the engine's action a trained engineer can determine its ailments as 
surely as a skilled physician can detect the weaknesses of a patient's 
heart action by the aid of a stethoscope. Every deviation of the 
curve from the standard form means to this expert a fault either of 
design or of adjustment. Poor lubrication, late admission of the 
steam, excessive back pressure, too early cut-off, etc., each makes 
its impression on the curve, and each trouble in turn can be cor- 
rected and proof given that this has been done by noting the 
improvement in the curve on a new indicator card. 

<I In addition to this information, a measurement of the area of the 
diagram, together with known constants of the engine and indicator, 
enable one to determine the exact number of horsepower produced 
by the engine. 

<I Another important adjunct of the modern engine is the " Valve 
Gear", by which the admission of the steam to the cylinder, the cut- 
off, the expansion, compression, and exhaust are controlled. The 
proper operation of the valves of an engine is of the highest economic 
importance and not only must the expert engineer understand the 
working theory of this control device and understand the differences 
between a Stephenson, Walschaert, or Reynolds-Corliss, for example, 
but he must be able to determine whether the valve actions are as 
perfect as they can be made by proper adjustment. By use of a 
graphical method called a "Zeuner Diagram", it is possible to 
determine the proper lap, lead, angle of advance, cut-off, and release, 
and to correct any errors of adjustment that may exist. 

<I All of these important matters in connection with the steam 
engine are carefully and authoritatively treated in this book in an 
exceedingly practical way. A number of examples taken from 
actual operation experiences are carefully worked out as a guide to 
the proper method of applying both the indicator and Zeuner 
diagrams. 



CONTENTS 

PART I 
STEAM ENGINE INDICATORS 

PAGE 

Types 2 

Watt indicator 2 

Crosby indicator 3 

Tabor indicator G 

American Thompson indicator .. . . . S 

Indicator spring testing 11 

Apparatus 11 

Spring calibration 12 

Engine connection : . 13 

Continuous diagrams 19 

Reducing motions 21 

Simultaneous indicator cards 28 

Detent attachment 29 

Assembling and adjusting indicator 30 

Assembling Crosby indicator 30 

Testing action 31 

Adjustment 32 

Taking cards 33 

Condition of indicator 33 

Sample indicator card 34 

Indicator card analysis 34 

Physical theory 41 

Pressure 42 

Work : 42 

Heat 43 

Horsepower 44 

Piston displacement 48 

Properties of steam ." 48 

Saturated vapor '. 48 

Steam tables 50 

Kinds of steam 51 

Feed water temperature 56 

Calorimetric measurements 57 

Volume and weight of steam 61 

Thermal efficiency 63 



CONTENTS 

PAGE 

Interpretation of indicator cards 64 

Theoretical diagram 64 

Steam cards showing miscellaneous troubles 66 

Gas engine cards 73 

Cards showing valve troubles 73 

Testing steam engines 75 

Factors considered 76 

Thermometers 77 

Indicators 77 

Scales 77 

Meters 77 

Gauges 78 

Calorimeters 78 

Prony brakes 78 

Speed counter 83 

Indicator troubles and remedies 84 

Necessity for care in using indicator 84 

Attachment of indicator 84 

Reducing motions 86 

Drum spring tension 86 

Adjustment of guide pulley 86 

Adjustment of pencil pressure 87 



PART II 



VALVE GEARS 

Valve characteristics 1 

Function 1 

Eccentric 2 

Valve motion . 3 

Lead 9 

Analytical summary of valve terms 11 

Valve diagrams 17 

Zeuner diagrams 17 

Illustrative problems 23 

Effect of changing lap, travel, or angular advance 29 

Design of slide valve 31 

Area of steam port 31 

Width of steam port 33 

Width of exhaust port 34 

Width of bridge 34 

Point of cut-off 34 

Design of slide valve 

Lead 35 



CONTENTS 

PAGE 

Design of slide valve (continued) 

Illustrative problem 35 

Reversing simple engine 37 

Valve setting 41 

Possible adjustments 41 

To put engine on center 41 

To set valve for equal lead 43 

To set valve for equal cut-off 44 

Modifications of slide valve 46 

Balancing steam pressure 46 

Reversing mechanism 50 

Shifting link type of valve gear 53 

Stephenson link motion 53 

Gooch link 63 

Radial type of valve gear , 64 

Hackworth gear 64 

Marshall gear 66 

Joy gear 67 

Walschaert gear. 67 

Double valve gears 73 

Meyer valve 73 

Shifting eccentric valve gear 78 

Thompson automatic valve gear 81 

Drop cut-off gears 85 

Reynolds-Corliss gear 86 

Nordberg gear 89 

Brown releasing gear 90 

Greene gear 91 

Sulzer gear 92 

Corliss valve setting 92 

Adjusting steam lap 93 

Adjusting exhaust clearance and lead f 94 

Adjusting cut-off 94 

Valve gear troubles and remedies 95 

Duplex pump valve gear 96 

Plain slide valve gear 97 

Corliss valve gear 100 

Stephenson valve gear 101 

Walschaert gear 103 



PART I 

STEAM ENGINE INDICATORS 

INTRODUCTION 

The steam engine indicator is an instrument designed to make 
an accurate graphical diagram of the pressure of the steam in the 
engine cylinder at all points of the stroke. This diagram affords a 
means for studying the performance of the steam engine. . 

The indicator serves two very important purposes, although 
manv other results are obtained by its use. (1) In the hands of an 
experienced engineer, it enables him to discover any defects in the 
design or setting of the valve mechanism. (2) It also indicates 
whether the steam ports are large enough and, in fact, a proper inter- 
pretation will disclose the exact condition of the design and operation. 
Thus the engineer can determine whether any change in the 
operation of the moving parts is advisable. 

The information that may be obtained by an intelligent use ot 
the indicator is of very great value to the engineer. The power of 
the engine at any time and under any condition may be deter- 
mined- manv facts can be accurately obtained that could not be 
secured in any other way; many things about the steam engine 
that before seemed mysterious are now made clear. Its value is so 
universally recognized that almost all builders of steam engines 
applv indicators to their engines and adjust the valves and moving 
parts before sending the engine away from their factories. For 
these and other reasons which might be mentioned, it is seen that 
the indicator has played no small part in the development of the 

steam engine. 

In the earlv development of the steam engine by James Watt, 
he realized that some means should be provided whereby the internal 
action of the steam, valves, etc., could be watched or their behavior 
interpreted. As a result of this apparent need, the indicator was 
devised, the first forms being crude in their construction but the 
underlying principles being the same as are found today in the mod- 
ern instrument. It is, therefore, of interest to note that the changes 



2 STEAM ENGINE INDICATORS 

made in the indicator since its advent have been largely in construc- 
tional details rather than in principle. The moving parts of the 
earlier indicators were exceedingly heavy; on this account, the 
inertia of the moving parts often distorted the indicator diagram to 
such a degree that the results obtained were unreliable. The older 
types would give fairly accurate results on slow-speed engines but 
were useless on high-speed engines on account of the comparatively 
great weight of the pencil mechanism and other moving parts. 

The modern indicator is almost perfect in construction. All 
of its parts are as light as good design will permit and it is conven- 
iently manipulated and easily adjusted. It may and does at times, 
however, record pressures incorrectly. Some of the most common 
errors, which are often misleading, will be discussed later. 

In order to have an intelligent understanding of the use and 
care of an indicator, it is necessary to become familiar with its 
construction, and to that end, a description of three well-known 
makes will be given, viz, the Crosby, Tabor, and Thompson. 

TYPES 

It will be observed that indicators do not differ in detail very 
materially, their chief difference being found in the pencil mechan- 
ism. In order to make a discussion of the construction logical in 
development, it is well to note first an improved form of the Watt 
indicator. 

Watt Indicator. The Watt indicator, Fig. 1, consists of a steam 
cylinder S, about 1 inch in diameter and 6 inches long, in which a 
solid piston P is accurately fitted. A spiral spring A is attached to 
this piston, and controls the motion of a pencil a, which is also 
attached to the piston. This pencil can operate on a sheet of paper 
fastened to a sliding board B. This board moves back and forth 
by means of a weight at one end and a cord at the other which is con- 
nected to some reciprocating part of the engine. The indicator 
cylinder S may be put in communication with the engine cylinder 
by means of the cock C. With this instrument, a complete diagram 
can be taken. When cylinder S is put in communication with the 
engine cylinder by means of the cock C, pencil a is raised or lowered 
precisely as the intensity of the pressure in the cylinder varies. This 
variation of height of pencil or pressures is registered upon a card 



STEAM ENGINE INDICATORS 



carried by the board B. As the board B is moved in exact coinci- 
dence with the piston of the engine, by being connected to some 
reciprocating part, the resulting card gives an exact indication of 
the pressure in the cylinder for all points of the stroke. The verti- 
cal dimensions of the card, commonly called the ordinates, indicate 
the pressures; the horizontal dimensions, or abscissas, indicate the 
simultaneous positions of the piston. 

It is a natural transition from the earlier form shown in Fig. 1 
to the modern indicator, as the intervening changes have been largely 
in the perfection of the recording mechanism and in the refinement 
of details, as will be pointed out later. 

Crosby Indicator. The Crosby indicator 
is illustrated in cross-section in Fig. 2. The in- 
dicator cylinder 4 is connected to the steam 
engine cylinder by means of the loose nut 7. 
The steam passes from the steam engine cylin- 
der to the indicator cylinder through the pas- 
sage 6 and acts on piston 8. The indicator 
cylinder is very carefully designed and con- 
structed so that the piston will have perfect 
freedom of movement for various pressures. 
The annular cavity between 4 and 5 serves 
as a steam jacket and permits 4 to expand 
and contract freely. 

Piston 8 is made of a good quality of steel 
and is hardened to prevent its surface from 
wearing. It is J square inch in area. Small grooves around its 
outer surface provide a steam packing, and the moisture and oil 
which collect in these grooves prevent too much leakage of steam 
past the piston. At the center of the piston is a boss or hub which 
projects both upward and downward. The upper part of the hub 
is threaded inside to receive piston rod 10. The upper edge of 
this hub is so formed that it fits nicely into a circular groove in 
the bottom side of the nut of the piston rod. The hub also has a 
slot cut diametrically across it, which permits the flat portion of 
the spring with head to fit on a curved bearing on the piston screw 9. 
When making connection between piston 8 and piston rod 10, it is 
very essential that the hub shank fit tightly against the bottom of 




Fig. 1. Original Watt 
Indicator 



4 STEAM ENGINE INDICATORS 

the circular groove in the bottom of the shoulder of the piston 
rod. If this connection is correctly made, a perfect alignment of 
the piston is assured. 

The swivel head 11 is threaded at the bottom, so that it can be 
screwed into the piston rod. By so doing the height of the pencil 
and, therefore, the atmospheric line can be raised or lowered as 
desired. 

Cap 2 is an important part of the indicator as it holds all the 
moving parts in place and guides the piston. It has two projections 




Fig. 2. Part Section of Crosby Indicator 



of different diameter on the lower side. The projection with the 
larger diameter is threaded so that the cap can be screwed into the 
cylinder. The smaller projection is also threaded to engage with 
like threads on the spring head which holds it firmly in position. 
Cap 2 holds sleeve 3 in position in a recess formed for the purpose. 
This sleeve carries the pencil mechanism, parts of which are 15, 13, 
etc. The arm X, which carries lever 15 of the pencil mechanism, 
is made integral with the sleeve. A handle 22 is provided by which 



STEAM ENGINE INDICATORS 5 

the pencil point is brought in contact with the paper. This handle 
is threaded and, being in contact with a stop screw on plate 1, 
permits a very delicate adjustment of the pencil point to the sur- 
face of the paper on the drum. It is desired to have the pressure 
just great enough (but no greater) to secure a visible diagram on 
the paper. 

The pencil mechanism, consisting of links 13, 14, lo, and 16, is 
a very important part of the indicator. Its essential kinematic 
principle is that of a pantograph. This mechanism must be so 
designed and adjusted that the path of pencil 23 is at all times paral- 
lel to the path of piston 8. The links are so proportioned that the 
movement of the pencil is six times that of the piston. 

The indicator card is held on paper drum 24 — which is made of 
very light metal — by means of clips 25 and 26. Drum 24 fits on a 
base 27, which carries a spring 31 on a central projection 28; this 
spring brings the drum back to its initial position when the indicator 
is detached from the moving parts of the engine or when 
a return stroke is made. 

The direction in which the cord may be conducted 
from the drum can be adjusted by means of guide pulleys 
36 and 37, which are attached to the indicator by nut 39 
and frame 33. 

The piston spring, Fig. 3, which should be of a good 
quality of spring steel, must be carefully made and tested, 
and also carefully handled, as the accuracy of the results 
depends in a large measure upon the accuracy of the 
spring. 

It will be noted in Fig. 2 that the piston or pressure spring is 
placed within cylinder 4, when the indicator is put together for use. 
This brings the spring in contact with the live steam and, as a con- 
sequence, errors may be recorded due to the uneven heating of the 
spring and contained parts. To eliminate the possible inaccuracies 
due to heat, the manufacturers have constructed indicators with the 
spring on the outside, as illustrated in Fig. 4. Aside from the elimi- 
nation of errors by the use of the outside spring, convenience is 
obtained in that the spring is more accessible and may be removed 
or changed without taking the indicator apart, which can not be 
done with the spring inside. Furthermore, the spring does not get 




STEAM ENGINE INDICATORS 



very much warmer than the surrounding atmosphere, so it is not 
necessary to allow the indicator to cool before removing it. 

Fig. 4 also shows how an indicator may be easily changed from 
an ordinary steam indicator to an indicator suitable for gas engine 
work. The change is made by reducing the size of the cylinder to 
J square inch in area and increasing the strength and weight of the 
pencil mechanism. By these changes, an indicator may be used on 
gas engines with good results. 




Fig. 4. Crosby Indicator with Outside Spring 



The Crosby indicator is ordinarily made with a drum 1| inches 
in diameter, this size being suitable for high-speed work. If, how- 
ever, a larger diagram is desired and the speed is low, a drum 2 
inches in diameter can be furnished. 

Tabor Indicator. The Tabor indicator, Fig. 5, with outside 
spring, reducing motion, and electrical attachment, is similar in 
construction, operation, and essential characteristics to the Crosby 
and other indicators, though there are details of design for which the 
respective makers claim advantage over other makes. One feature 



STEAM ENGINE INDICATORS 



of the Tabor which is essentially different from the Crosby and the 
Thompson is its pencil mechanism. As was shown in Fig. 2, the 
Crosby pencil mechanism consists of a system of levers, which gives 




Fig. 5. Tabor Indicator with Outside Spring 

to the pencil a straight-line motion parallel with that of the piston, 
with possible slight errors, especially on high cards. As shown in 
Fig. 6, the scheme for obtaining the parallel 
or straight-line movement of the Tabor indi- 
cator is different from that of the Crosby. It 
has the connecting links corresponding to 
13, 14, and 16 in the Crosby, Fig. 2, but 
in place of link 15, there is substituted an- 
other arrangement. A stationary plate 1, 
with a curved slot 2, is fastened in an up- 
right position to the cap. On the pencil bar 
is a roller bearing 3, which is secured to the 
bar by a pin. This roller moves freely in 

the curved slot in the guide and controls the motion of the pencil 
bar. The position of the slot and the guide upright is so adjusted 




Fig. 6. Tabor Device for 
Straight-Line Movement 



8 



STEAM ENGINE INDICATORS 



and the guide roller is so placed on the pencil bar that the curve of 
the guide slot controls the pencil motion and absolutely compen- 
sates for the tendency of the pencil to move in a curve. There 
is a minimum of friction in this movement and guide, and no dis- 
turbance from inertia has been detected by the most careful tests. 

American Thompson Indicator. The American Thompson 
indicator, Fig. 7, does not differ greatly in general appearance from 




Fig. 7. American Thompson Indicator 

other indicators, but a close comparison will show some difference 
in the details of construction. For. instance, it is evident that the 
arrangement of the levers, which make up the pencil motion, differ 
slightly from that of the Crosby indicator. Each maker, of course, 
makes the assumption that his particular arrangement is the best. 
In most cases, the purchaser must use his own judgment in the matter. 
Again the connection of the spring to the piston is different on the 



STEAM ENGINE INDICATORS 

TABLE I 
Constants of Indicator Springs 



Scale 

of 

Springs 


Maximum Safe Pressures to Which Springs Can Be 
Subjected 


Pounds Pressure per Square Inch with J-£ Square 
Inch Area Piston 


To 200 

Revolutions per 

Minute 


To 300 

Revolutions per 

Minute 


8 

10 

12 

16 

20 

24 

30 

32 

40 

48 

50 

60 

64 

80 

100 

120 

150 

200 


10 

15 

20 

28 

40 

48 

70 

75 

95 

112 

120 

140 

152 

180 

200 

240 

290' 

375 


6 

10 

15 

22 

32 

40 

58 

62 

80 

95 

100 

115 

125 

145 

160 

195 

250 

330 



Thompson than on others, in that the spring screws directly into an 
enlargement on the upper side of the piston, thus being rigidly 
attached to the piston as well as to the pencil motion at the top 
instead of having a semi-flexible connection by means of a ball and 
socket joint, as in the Crosby. These two points are the distin- 
guishing ones of the Thompson indicator. The construction of its 
cylinders, piston, paper drum, etc., are about the same as for those 
previously described. In the figure, a portion of the drum is shown 
cut away, disclosing the detent motion, the operation and purpose 
of which will be described later. 

The piston of an indicator is usually .798 inch in diameter, which 
is equivalent to J square inch area. This size piston with springs is 
designed to indicate pressures up to 250 pounds. When higher 



10 STEAM ENGINE INDICATORS 

pressures than 250 pounds are used, a piston .564 inch in diameter, 
representing an area of I square inch, is substituted for the ^-inch 
piston. This doubles the capacity of the spring and makes it pos- 
sible to indicate up to 500 pounds. 

Since it is the capacity of the spring that limits the height of 
the indicator card and since the various springs are made to resist 
a definite amount of pressure, it is necessary that the proper capacity 
of spring be used at all times. This capacity is designated by the 
term "scale of spring" which means the amount of pressure required 
on the piston per square inch of area to raise the pencil point 1 inch. 
For example, if one hundred pounds steam pressure is being used and 
a spring having a scale of 40 is placed on the indicator, the height of 
the resulting card will be 100^-40 = 2§ inches. The capacity of 
the spring is always marked upon it, as 40, 60, etc. The manu- 
facturers of the Tabor indicator recommend the use of springs hav- 
ing capacities for various conditions of speed and pressure as given in 
Table I. 

If an engine is running at a speed not exceeding two hundred 
revolutions per minute and the steam pressure being used is 180 
pounds, the scale of spring to be used is 80. If the revolutions per 
minute (r. p. m.) be increased to between two hundred and three 
hundred, then a spring of 80 pounds should not be used for pressures 
higher than 145 pounds. This table is about the same as that given 
by other makers of indicators. A common rule for determining 
the capacity of the spring to be used is to multiply the scale of the 
spring by 2\ and subtract 15, the result being the limit of pressure 
to which the spring should be subjected. To illustrate: Assume a 
spring having a scale of 60. Then 60X2^ = 150. 150 — 15 = 135. 
Therefore, 135 pounds pressure is the ultimate capacity of a 60-pound 
spring, which approximately checks Table I. 

From the foregoing discussion of the indicator and the study 
of its construction, it is evident that the essentials of a good indi- 
cator are summed up by Professor Thurston in the following para- 
graphs : 

(1) Such form and construction as will insure its meeting the prescribed 
general conditions — accuracy of representation of variations of steam pres- 
sure and simultaneous movement of the piston at all times. 

(2) Such simplicity of form as will make it free from liability to accident 
and failure in operation. 



STEAM ENGINE INDICATORS 



11 



(3) Such lightness of parts and rigidity of whole, as will prevent any 
inaccuracies of indications arising from its inertia. 

(4) It should be easily, conveniently, and safely attachable and unmov- 
able and handily manipulated. 

(5) Stiffness, lightness, and exactness of standardization are prime 
essentials. Springs should be exactly standard. Moving parts as light as 
consistent with proper strength and stiffness; stationary parts should be 
carefully proportioned and rigid. 

INDICATOR SPRING TESTING 

Apparatus. As the accuracy of the action of the indicator 
spring is of primary importance in obtaining correct indications, 




Fig. 8. Spring Testing Device 



some means must be employed for testing indicator springs. A very 
simple but efficient apparatus for testing indicator springs is shown 
in Fig. 8, which consists of a drum A, made of 4- or 5-inch extra 
heavy pipe having steam-tight joints. Steam is admitted to the 
drum at B, and permitted to pass out at C, through a piston regulat- 
ing valve D, carrying a disk and weights. 

The indicator and a standard test gauge for checking are attached 
in the manner shown. The pressure regulating valve D is very sen- 
sitive and responds to a very slight change of pressure. By plac- 
ing the desired weight on the disk and adjusting valve B, the pres- 
sure of steam in the cylinder is maintained at a constant value. If 



12 



STEAM ENGINE INDICATORS 



the pressure should rise higher than desired, the piston valve D 
rises, permitting the escape of steam through pipe C, and in this way 
maintains a constant pressure in the drum. 

Spring Calibration. To test or calibrate the spring, proceed 
as follows: Put the indicator together properly and see that the 




*?OLB. SPRING 

ao 



5013. SPRING 
/OO 



60LB.SPRJNG 
/20 



GOLB. SPRING 

teo 



IOOLB. SPP/NG 
300 



Fig. 9. 



1 20 LB. SPRING 
2*0 




o o 

Cards Showing Records of Spring Tests 



/50LB. SPR//YG 
300 




piston is oiled and in place. Attach the indicator in the usual man- 
ner. After the indicator has been warmed up by permitting steam 
to act on it, put the desired weight on the disk and spin it, at the 
same time moving the indicator drum by pulling the cord and hold- 



STEAM ENGINE INDICATORS 



13 



ing the pencil against the paper drum, thus recording the pressure 
on the paper. Proceed in this manner by equal increments of 
pressure until the capacity of the spring has been reached, then 
reduce the pressure by the same increments until zero is reached. 
The operation of taking both the upward and the downward read- 
ings should be continuous, stopping only long enough to change the 
weights and to make the proper indications. 

Measuring the Cards. After the cards have been taken, rhey 
may be measured by means of a scale in the usual manner. The 
pressures thus measured should check within a fairly close margin 
of the readings corresponding to the gauge and the tester. 




Fig. 10. Engine Showing Two Indicators Screwed Into Cylinder 

The cards, Fig. 9, show the records obtained from tests of 
various springs. It is evident that some of the records taken with 
increasing pressures do not coincide with the corresponding record 
when going down or with decreasing pressures. For very accurate 
work, the spring should be used with the piston and in the indicator 
with which it was tested. 

Engine Connection. The attachment of the indicator to the 
engine should be such that the pressure of the steam on the indicator 
piston is exactly the same as that acting at the same instant on the 
engine piston. In order to secure this result, the steam connection be- 
tween the indicator and the engine should be amply large and direct. 



14 



STEAM ENGINE INDICATORS 



If possible, the indicator connection should be screwed directly into the 
cylinder as shown at A, Fig. 10. In making this attachment, a hole 
is drilled in the cylinder and a connection is made to the indicator 
by means of a standard J-inch pipe and a proper valve or cock. 
The hole in the cylinder should be drilled in the clearance space, 
where the piston will at no time cover any portion of the opening, 
and where no strong currents of steam will sweep directly into the 
passage. 

If it is possible to remove the cylinder heads, it should be done 
before drilling, so as to properly locate the holes and to remove any 
chips which may happen to fall into the cylinder while it is being 





""^ 


R "~fM 




BPu^ ^^'-* m H 




<»] wk * 4»i r^ 


W^SmMJ; 





Fig. 11. Method of Attaching Indicator to Locomotive 

drilled. If it is not feasible to remove the cylinder heads, the cylin- 
ders should be carefully blown out with steam before running the 
engine, as much damage may result from the chips in the cylinder 
The indicators should be attached in an upright position if possible. 
It is best to have an indicator attached to each end of the cylinder, 
so that cards may be taken simultaneously from both ends. Before 
drilling the holes, a general plan or scheme should be studied out for 
the attachment of the indicator and its necessary appliances, as the 
type of engine (whether vertical or horizontal), the type of cross- 
head, and the necessary room for operation may be quite different 
for each case; so a strict rule can not be laid down for this procedure. 



STEAM ENGINE INDICATORS 



15 



Suffice it to say that generally the indicator can be attached to the 
side of the cylinder or to the top, as shown in Fig. 10. Figs. 10 and 
11 illustrate the methods used in attaching indicators to a simple 
engine and a locomotive, respectively. It will be observed that all 
of these connections are short and direct, that the indicators are in 
an upright position, and that the cord of the indicator is led straight 
to the crosshead connection. 

Sometimes it is not convenient to use two indicators or it may be 
that the engineer does not care to bear the cost of two, so only one is 
used. When only one is used, a pipe leading from each end of the cyl- 
inder is connected to the indicator by means of a three-way cock, as 



W/TH FEGULAR ELBOWS 




Fig. 12. Typical Three-Way Valve 

shown in Fig. 12. By the use of this cock, the indicator is put in con- 
nection first with the head end (h. e.) and then the crank end (c. e.) of 
the engine. This should be done with as little loss of time as possible 
so that the cards will represent, as nearly as possible, actual conditions 
of pressure in the cylinder. The three-way cock, or any other cock 
which may be used in the system, should have an opening as large as 
that in the cylinder connection of the indicator. It should also have 
a hole in one side for the purpose of freeing the indicator and its 
connections from any water that may come over with the steam. 
Avoid Long Pipe Connection. Long connections between the 
indicator and the engine cylinder should be avoided in all cases. 



16 



STEAM ENGINE INDICATORS 



Experiments conducted at Purdue University have demonstrated 
the fact that any pipe connection between the indicator and the 
engine is likely to affect the action of the indicator. Under ordi- 




Fig. 13. Method of Attaching Indicator Piping to Engine Cylinder 



nary pressures and speeds, a length of pipe over 3 feet in length ?o 
distorts the card that the results obtained are useless except for 
approximate work. 



■P/P£ CARD 
<CYJ-/NDER CARD 




Fig. 14. Indicator Card Showing Effect of 
Changing Connecting Pipe 



In order to determine the effect of long pipe connections between 
the indicator and engine, upon the form of the cards, a series of tests 



-P/PE CARD 

>CYA./HDER CARD 




Fig. 15. Indicator Card Showing Effect of 
Changing Connecting Pipe 



were conducted in the Engineering Laboratory of Purdue Univer- 
sity under the direction of Dean W. F. M. Goss. The results of 
these experiments formed the basis of a paper which Dean Goss 



STEAM ENGINE INDICATORS 



17 



presented before a meeting of the A. S. M. E. at St. Louis in May, 
1896. The experiments were made in connection with a 7f- by 
15-inch Buckeye engine. Very great care was taken in the selec- 




Fig. 16. Indicator Card Showing Effect of 
Changing Connecting Pipe 



tion and testing of the indicators and in their manipulation, in order 
to insure that any distortion which might occur in the cards would 
be due entirely to the pipe connections. The indicators were 




Fig. 17. Indicator Card Showing Effect of 
Changing Connecting Pipe 



attached to the cylinder, as shown in Fig. 13, both being connected 
at the same end so that the indicator pistons would be exposed as 
nearly as possible to identical conditions. 




Fig. 18. Indicator Card Showing Effect of 
Changing Connecting Pipe 



The indicator A and the cards obtained therefrom will be here- 
inafter designated as cylinder indicator and cylinder cards and it is 



18 



STEAM ENGINE INDICATORS 



assumed that this indicator will give indications true to the condi- 
tions in the cylinder. 

The indicator B and the cards obtained therefrom will be desig- 
nated as the pipe indicator and pipe cards, respectively, and it is 
assumed that any perceptible difference in the cards obtained from 




-+TP/PE CARD 

-CYLINDER CARD 



Fig. 19. Indicator Card Showing Effect of 
Changing Connecting Pipe 

the cylinder indicator and from the pipe indicator will be due entirely 
to the pipe connections. 

Pipe connections of 5, 10, and 15 feet were used, the length of 
pipe being measured from outside of the cylinder walls to the end of 
the coupling under the indicator cock. Care was taken in securing 
easy bends in the pipe so as not to retard the action of the steam. 




Fig. 20. Indicator Card Showing Effect of 
Changing Connecting Pipe 

The pipes were also properly insulated in order to avoid in so far as 
possible any condensation. 

The method of procedure was to run the engine for a short 
length of time, until the desired speed, cut-off, pressures, etc., were 
obtained, then cards were taken simultaneously from the two indi- 
cators. Two cards were taken from each indicator, then the indica- 
tors were interchanged and two more cards taken from each, thus 
obtaining four cylinder cards and four pipe cards. 

Figs. 14 to 22 inclusive illustrate the effect of the pipe on the 
form of the indicator card with the engine running under various 



STEAM ENGINE INDICATORS 



19 



conditions. A study of these cards reveals the fact that the length 
of the piping to the indicator affects very materially the area of the 
diagram. The events of the stroke, while remaining unchanged, 
are apparently generally made later by the use of the long pipe, 
but in some cases, some of them are earlier. 




Fig. 21. Indicator Card Showing Effect of 
Changing Connecting Pipe 

It is hoped that what has been said is sufficient to point out the 
importance of the short indicator connection. 

Continuous Diagrams. From the study of the indicator, it 
has been obvious that the indicator card gives an indication of what 
is taking place in the cylinder at a specific moment. This being the 
case, it is practically impossible to obtain by its aid determinations 




Fig. 22. Indicator Card Showing Effect of 
Changing Connecting Pipe 

that are to be relied upon when the engine is working under con- 
stantly varying conditions, as in gas engines, locomotives, marine 
engines, and rolling-mill engines. To meet this demand, an attach- 
ment has been developed whereby it is possible to take a continu- 
ous card, thus getting exact determinations under the most variable 
conditions. 

Crosby Device. Fig. 23 represents a Crosby indicator equipped 
for taking continuous diagrams. The special drum is designed so 
as to be applied to any Crosby indicator, and uses a roll of paper 



20 



STEAM ENGINE INDICATORS 



2 inches wide and 12 feet long upon which the series of diagrams 
are traced. The roll of paper is located within an opening in the 
drum. From the roll, the paper passes around the outside of the 




Fig. 23. 



Crosby Indicator with Continuous 
Diagram Device 



drum, thence inward to a central cylinder to which it is attached. 
In taking cards the paper rolls up on the central cylinder, which is 
concentric with the drum, and may be withdrawn through the top 
and easily detached. On the top of the drum is a knurled head, 




Fig. 24. Continuous Diagram from Rolling-Mill Engine 



loosely attached to the drum spindle, which controls the distance 
between diagrams. Adjustment can be made so that from 6 to 
100 diagrams can be made per foot of paper. 

Fig. 24 illustrates a series of continuous cards taken from a 
rolling-mill engine and clearly shows the widely varying conditions. 



STEAM ENGINE INDICATORS 



21 



Fig. 25 shows cards taken from an automatic cut-off rolling- 
mill engine. 

After providing proper means for attaching the indicator to 
the cylinder, the next important step is to provide a convenient and 
at the same time correct reducing or drum motion. 




Fig. 25. Continuous Diagram from Automatic Cut-Off Rolling-Mill Engine 



Reducing Motions. In the description of the indicator, it 
was noted that the indicator card is held on the circumference of the 
paper drum by means of clips. Since the circumference of the drum 
is much less than the length of stroke of the engine, some means 
must be provided to reduce the motion of the drum. As each 
engine and its location will be different, no strict rule can be given 
whereby one can at once provide a reducing motion, but each case 
must be studied and the best means possible provided to meet the 
exigency. A few examples and principles will be given and doubt- 
less they will suggest others to meet 
specific cases. 

Brumbo Pulley. The Brumbo pul- 
ley, Fig. 26, is easily and quickly made 
and can be attached to almost any en- 
gine. If it is to be used for only a short 
time, it may be constructed of wood, care 
being exercised in having close fitting 
joints. If the engine is to be indicated 
frequently, it is better to make the re- 
ducing motion of metal in order that the 
wear in the joints may be minimized. 
The reducing lever A is pivoted overhead 
to some temporary support or, if it is 
to be permanently attached to the engine, some permanent sup- 
port, such as an upright post or bracket, may be attached to the 
frame of the engine, as in Fig. 10. The segment S is made fast to 
the lever A, so that its semicircumference is true with its pivot 





Fig. 26. Brumbo Pulley 
Reducing Motion 



22 



STEAM ENGINE INDICATORS 



point B, upon which the lever swings. The sector may be set at 
any angle with the lever. The lower end of the lever A is attached 
to the crosshead through the link C. The length of the lever A 
should be at least one and one-half times the length of stroke of 
the engine. The length of the connection C may be about one- 
half of the length of stroke, but it may be greater. When the cross- 
head is at its mid-position, the lever A should be vertical. Dur- 
ing the stroke of the engine, the link C should swing equally above 
and below a horizontal position. 

With this form of reducing motion, the cords may be led in 
any direction in a vertical plane from the sector and one or more 
cords may be led off to different indicators. The face of the sector 
should be true and the radius must have a value sufficient to give 

the required motion to the drum. 
To fulfill this condition, the radius 
of the sector must bear the same 
relation to the length of the lever 
A as the proposed length of the 
indicator diagram bears to the 
stroke of the engine. 




Fig. 27. 



Diagram Showing Principle of 
Brumbo Pulley 



Example. Suppose it is desired to 
make a reducing motion for a 10- by 
16-inch engine. Assume the length of 
the lever L to be 24 inches and the re- 
quired length of card D to be 4 inches. The length of stroke S is 16 inches. 
Designating the radius of the sector as R, then 

R.L:: D:S 
Solving for R in the above engine 

R: 24:: 4: 16 
16 R =96 

R = 6 inches 

The principal objection to the Brumbo pulley is that it is not 
interchangeable, that a different one is required for engines of dif- 
ferent types and sizes. If it is carefully made and attached, how- 
ever, it will give results with very slight inaccuracies. The design 
illustrated in Fig. 27 is theoretically correct, and its construction 
and operation are so clearly shown that a detailed description is not 
deemed necessary. It has been successfully used for experimental 
work in colleges and universities tor a number of years. 



STEAM ENGINE INDICATORS 



23 



Pantograph. Another form of reducing motion, known as the 
pantograph, is shown in Fig. 28. It is placed horizontally, with the 
pivot B resting on a support opposite the crosshead when in mid- 
position. The pivot A is attached to the crosshead, usually by hav- 
ing the stud A inserted in a hole drilled in the crosshead. If the 
pivot B is adjusted to the proper height and at the right distance 
from the crosshead, the cord from the indicator may be attached 
to the pin E without any pulleys, which is very desirable. The 
length of the diagram is adjusted as desired by means of the movable 
piece C D and the pin E. The pin E must always be on a line join- 
ing the pivot points A and B. The pantograph gives correct 
results when in good condition and properly attached but, on account 
of the large number of joints, 
it may become unsatisfactory. 
This type is usually used on 
engines having a long stroke 
and where it is not convenient 
to attach a Brumbo pulley or 
its equivalent. It is applicable 
to all types of engines of any 
length of stroke. In two of 
the three forms of reducing 
motions just described, there 
are chances for inaccuracies. 
There is an error in the Brum- 
bo pulley which may or may not be very large, depending on the pro- 
portions of the levers, and there may be lost motion in the many joints 
of the pantograph. The inaccuracy of the Brumbo pulley can be mate- 
rially decreased by using, instead of the sector, a sliding bar, as in 
Fig. 11. This sliding bar is supported by means of brackets and is 
attached to the lever by means of a pin which works in a slot similar 
to that at the lower end of the lever. As the crosshead moves to and 
fro, the sliding bar likewise moves, and its motion is proportional to 
that of the crosshead at all points of the stroke. All things being 
considered, the principle of the reducing motion shown in Fig. 11 
is all that could be desired; it is especially suited for locomotive 
engines. 

Reducing Wheel Nearly all makers of indicators manufacture 




Fig. 



28. Pantograph Device for 
Reducing Motion 



24 



STEAM ENGINE INDICATORS 



a reducing wheel apparatus which serves the same purpose as the 
lever and pantograph types of reducing motions. A type of this 
apparatus is shown in Fig. 29; also it may be seen in Fig. 5 attached 
to the indicator. The following description is given by the makers. 
The reducing wheel is composed of a supporting base piece A, 
provided with short standards B that form bearings for the worm 
shaft on which the flanged pulley D is rotated, the outer bearing 
being a pivot which receives the entire thrust of the shaft, thus 
reducing the friction to a minimum. It is connected directly to the 
indicator upon the projecting arm that supports the paper drum, 
and the teeth of the worm shaft mesh directly into the teeth on the 

drum carriage C. Con- 
nected with the base piece 
is a spring case E, and on 
the extreme end of the 
worm shaft is secured a 
collar F through which 
freely slides a clutch pin, 
one end of which is se- 
curely fastened to a thumb 
piece for operating it. 

The flanged pulley D 
runs freely and independ- 
ently on the worm shaft, 
and has on its outside a 
clutch-shaped hub. To this pulley is connected the actuating cord 
G, which should encircle it a sufficient number of times to have its 
length, when unwound, a little more than equal the length of the 
stroke of the engine. The other end of the cord is secured to the 
crosshead of the engine or to a standard bolted thereto or to any 
moving part that has an exact similar motion, and must be con- 
nected in line from the pulley. 

Enclosed in the spring case E is a small, plain, spiral steel spring 
which operates solely to return the pulley to its starting point, after 
it has been revolved in one direction by the forward movement of 
the crosshead. As this pulley has an independent rotating back- 
and-forth motion on the worm shaft, the necessity of unhooking the 
cord when the indicator is not being operated is entirely overcome. 




Fig. 29. Tabor Reducing Wheel 



STEAM ENGINE INDICATORS 25 

The paper drum is rotated forward by means of the pulley through 
its worm shaft, engaging with the worm gear on the paper drum 
carriage, and is rotated in the opposite direction by the action of its 
own retracting spring. On top of the paper drum is a knurled thumb 
piece (see A, Fig. 5) made with a projecting pin on its under side to 
engage with a similar pin located in the top of the drum; this is to 
be used by the operator in moving the paper drum slightly forward, 
preparatory to taking a diagram, in order to prevent it from striking 
against its stop on the return motion. 

To operate this device, first, select a pulley whose diameter is 
about one-twelfth of the length of the engine stroke in inches. Prop- 
erly place this pulley upon the worm shaft by removing the clutch 
and then sliding the pulley onto the shaft, being particular that the 
small hole in the pulley brass disks sets over the projecting pin in the 
cover of the spring case. Then replace the clutch by pushing it onto 
the shaft as far as it will go, and secure it there by means of the set 
screw. 

Now place the indicator on the engine in such a position that 
the side of the pulley D will be parallel with the motion of the cross- 
head. Run out the loose end of the cord to a distance of at least 
12 or 18 inches beyond the extreme forward travel of the crosshead, 
still leaving a turn or two of the cord on the pulley unwound. While 
holding the cord, allow it to gradually recede and rewind itself on 
the pulley until its loose end has reached a point coincident with the 
extreme backward travel of the crosshead. If only a slight tension 
of the cord exists at this point, it will be sufficient, and the cord 
may then be attached to the selected point on the crosshead. The 
cord tension may always be adjusted either by winding the cord on, 
or unwinding it from, the pulley, as the case requires, one increas- 
ing and the other decreasing the tension. 

A much lighter cord can be used in proportion as the sizes of 
the pulleys increase. 

When the crosshead, with cord connected, is at its extreme for- 
ward travel, there should be just sufficient tension on the spring 
enclosed in the spring case to take up all slack of the cord when 
running, without overtaxing the spring. If, upon starting the engine, 
the cord should at first run unevenly on the pulley, turn the indi- 
cator to one side slightly until a perfect and uniform winding of 



26 STEAM ENGINE INDICATORS 

the cord is obtained, which can always be easily secured. When 
the pulley is running, motion to the paper. drum is obtained by push- 
ing in the swivel collar to which the clutch pin is secured. 

When ready to take diagrams, after placing the paper on the 
drum it is necessary first to advance the drum away from its stop 
fully J inch, which can be done by turning with one hand the knurled 
top thumb piece. WTiile holding drum in this position, with the 
other hand push in gently the swivel collar to start the paper drum 
in motion. The motion of the paper drum can at any time be 
stopped for removing diagrams taken and renewing the paper by 
withdrawing the swivel collar or by turning the top thumb piece, 
the latter method being preferable, as it prevents damage from too 
severe contact with the paper drum stop. The stopping of the paper 
drum will not affect the motion of the pulley, which will continue 
to revolve independently while the engine is in motion until the cord 
is disconnected. 

With the indicator are usually furnished three different size 
pulleys. Unless otherwise specified, pulleys furnished are 1, 2, and 
3^ inches in diameter. These pulleys are sufficient for the average 
work required of an indicator Larger sizes can be obtained if needed. 

The reducing wheel form of reducing motion has many points 
of advantage over the pantograph and lever in that it is conven- 
iently attached and it allows the operator to start and stop the 
motion of the paper drum without disconnecting the cord where 
attached to the crosshead, which is an annoying thing to do under 
some circumstances. The reducing wheel does not work under high 
speeds as satisfactorily as for the lower speeds, which is, perhaps, one 
of its most objectionable features. When the indicator is kept in 
constant use for several hours at a time, some trouble may be experi- 
enced with the cord becoming worn and breaking during a test. 

From this study of the reducing motion, it is evident that much 
must be left to the discretion of the operator as to the selection and 
attachment of the motion. 

Crosby Reducing Wheel. The Crosby reducing wheel, shown 
in Fig. 30, is attached directly to the cylinder cock of the steam 
engine, and has connected to it the steam engine indicator which 
it is to serve; thus it forms a base of support for the latter, and 
receives all the strains and shocks in the operation of the engine, to 



STEAM ENGINE INDICATORS 



27 



the relief of the indicator. Its bearings are made comparatively 
frictionless by the introduction of minute balls running in hardened 
tool steel races, thus affording lightness and freedom of movement. 
The cord pulley is horizontal, to allow the cord leading to the engine 
crosshead to take any direction the circumstances may require, 
without regard to the position of the indicator. 

Whenever the reducing wheel is to be attached to a vertical 
engine, an elbow nipple is provided, which will allow the cord pulley 




Fig. 30. Crosby Reducing Wheel Attached to Indicator 

to travel in the proper plane for guiding it to the crosshead of the 
engine with the indicator in an upright position as usual. 



OPERATION RULES 

The Crosby reducing wheel is attached directly to the cylinder cock of 
the steam engine by means of union 4 of standard 1 . Connect the indicator 
to standard 1 with the paper drum standing over spring 14 and the indicator 
guide pulley in a proper position over stroke pulley 20. 



28 



STEAM ENGINE INDICATORS 



To attach the cord guide: Loosen cord guide 24 by means of the screw 
beneath the cord pulley; then move it around to the proper position for the 
cord to pass directly through the hole in the cord guide without rubbing, to the 
crosshead of the engine and tighten it in place. 

To take up the tension spring: Release thumb screw 27 in the end of the 
shaft within spring tube 14; withdraw knurled spring head 16 from its square 
end, and turn it one or more squares as may be desired. 

To adjust the stroke pulley: Remove knurled disk 21, which holds in place 
stroke pulley 20 on the gear shaft; place on the shaft the stroke pulley desired; 
replace the disk and screw it up firmly. 

To attach the indicator cord: Wrap the indicator cord one or more turns 
around stroke pulley 20, passing the end through the hole in, and around, the 
hook of knurled disk 21. 

When used with other indicators, loosen bolt 3 in the side of standard /, 
where it is attached to the cylinder cock of the steam engine; remove the bush- 
ing and insert another fitted to the indicator to be used. 

Simultaneous Indica= 
tor Cards. In making 
complete and reliable 
tests of power plants, it 
is desirable that all of 
the cylinders of the com- 
pound and multiple en- 
gines be taken simulta- 
neously at a given signal. 
This requirement, if the 
indicators are hand-oper- 
ated, would necessitate 
an operator at each cyl- 
inder, which would be 
expensive and besides 
would not insure the 
simultaneous taking of 
the cards. The makers of indicators have met this need by sup- 
plying the market with an electrical attachment B, Fig. 5, which 
is attached to each indicator that is to be used. It is not thought 
necessary to give a description of the construction and operation 
of the appliance, but suffice it to say that by pressing an electric 
button, the pencils of all the indicators in the circuit are simulta- 
neously brought in contact with the paper and thus a record is 
made. 




Fig. 31. Indicator with Detent Attachment 



STEAM ENGINE INDICATORS 



29 



Detent Attachment. Another attachment that can be obtained 
and which is of much convenience to the operator at times is the 
detent attachment, shown in Fig. 31. It consists of a ratchet B 
that fits into the teeth of the wheel A. When the operator wishes 
to stop the motion of the paper drum, he pushes the lever C, which 
causes B to come in contact with A, as shown. Thus, the operator 
may change the card, and do other things without disconnecting 
the indicator cord from its crosshead connection. This is a very 
desirable attachment when indicating high-speed engines and when 
taking cards on a locomotive on the road where conditions are not 
ideal for using the indicator. 

Fig. 32, illustrates the detent attach- 
ment used on the Thompson improved 
indicator. With this new improved detent 
motion, in order to stop the paper drum it 
is only necessary to move lever A in the 
direction traveled by the paper drum until 
the drum releases itself. The lever must 
then be returned to its original position. 
When ready to take the diagram, turn 
forward the paper drum, by means of the 
milled rim B on top, until it catches, caus- 
ing the drum to revolve in the usual man- 
ner; then take the diagram and release the 
drum as described above. Before taking 
the diagram, see that the parts are cleaned 
and well oiled. To oil, remove the knurled 
nut F, take off the paper drum, then with 

the wire clip (which is sent with each indicator) remove the auxiliary 
spring case H by catching the end of the clip in the notches of the 
spring case, turning it forward until it releases from the catches; then 
move the spring and inner sleeve I. After cleaning and oiling, replace 
the inner sleeve I by inserting it into the drum so that the pin on the 
outside of the sleeve will enter the slot inside of drum bearing, and 
turn it until it comes to a stop; then with the wire clip catch hold 
of the auxiliary spring holder H and give the auxiliary spring E a 
tension of about one-fourth turn, and catch the points on the spring 
case H into the slots provided for them. 




Fig. 32. Thompson Detent 
Attachment 



30 STEAM ENGINE INDICATORS 

ASSEMBLING AND ADJUSTING THE INDICATOR 

Thus far, it has been the endeavor to mention the chief features 
of different makes of indicators and to point out the important 
points to be observed in the attachment of the indicator for obtain- 
ing a correct movement of the paper drum. Before proper results 
can be obtained, however, it is absolutely necessary that the indi- 
cator be properly put together. By way of illustration, reference 
will be made to Fig. 2, which shows the Crosby indicator with all of 
the parts connected together ready for connection to the engine 
cylinder. 

Assembling Crosby Indicator. When the indicator is removed 
from the engine cylinder, the spring should in every case be dis- 
connected from the piston and well cleaned before putting away. 
To remove the piston and spring, unscrew cap 2; then take hold 
of sleeve 3 and lift all the connected parts from the cylinder. This 
gives access to all the parts for the oiling and cleaning, 
which should be thoroughly done after each time the 
indicator is used. None of the pins as 17, 19, etc., 
should be removed except in making repairs and they 
should be kept well oiled in order to reduce friction. 
After removing the spring and cleaning the indicator 
thoroughly, connect the parts together, leaving out the 
"placing spring, and put the indicator away. It is evident that 
pnng ^ e indicator must be put together each time it is 
used and in doing this great care must be exercised in order to insure 
satisfactory working of the parts. It is important to notice that 
on the under side of the shoulder of the piston rod B, Fig. 33, there 
is a circular channel formed to receive the upper edge of the slotted 
socket of the piston A. 

Connecting Piston Rod. Whenever it is desirable to connect 
the piston rod with the piston, either in the process of attaching a 
spring, or for the purpose of testing the freedom of movement of the 
piston in the cylinder without a spring, be sure to screw the piston rod 
into the socket as far as it idll go; that is, until the upper end of the 
socket a is brought firmly against the bottom of the channel b in 
the piston rod. This insures a perfectly central alignment of the 
parts and, therefore, a perfect freedom of movement of the piston 
within the cvlinder. 




STEAM ENGINE INDICATORS 31 

Attaching Spring. To attach the spring, place the piston rod 
B, Fig. 33, in a hollow wrench provided for that purpose, so that 
the wrench will encircle the hexagonal part of the piston rod. Hold- 
ing the hollow wrench with the piston rod in place, in a vertical posi- 
tion, place the spring over the wrench so that round head 1 will be 
in the concave portion of the end of the piston rod. Unscrew set 
screw 9 until it is almost removed from the piston. Invert the 
piston and insert the transverse wire at the lower end of the spring 
in the slot in the socket of the piston. Screw the piston on to the 
piston rod as far as it will go. Remove the wrench, and holding 
sleeve 3 and cap 2 (see Fig. 2) in an upright position, so that the 
pencil lever will drop to its lowest position, engage the threads of 
swivel head 11 with those inside the piston rod, and screw it up 
until the threads on the lower projection of cap 2 engage those 
in the spring head. Continue the process until the spring is screwed 
up tightly on cap 2. After this, holding sleeve 3 in one hand, with 
the other turn the piston swivel onto the piston rod. It sometimes 
occurs that when the piston rod is screwed up on the swivel, the 
atmosphere line (the line inscribed by the pencil on the paper drum) 
is not properly located, so it becomes necessary to unscrew the pis- 
ton rod until the atmosphere line is at the proper height. A little 
practice will soon teach one about how the parts should be left in 
order to bring the atmosphere line in the correct position. It is 
important to avoid having the atmosphere line too high, as trouble 
might result if the pencil point moved above the top of the paper 
drum. 

Having thus secured the spring to the piston and cap, take the 
open wrench and turn set screw 9 snugly against the head on 
the spring. It is important that this should not be done until the 
spring has been securely fastened to the piston and to cap 2, for 
there is then less likelihood of error in alignment. After complet- 
ing the successive steps named, oil the piston with a good cylinder oil, 
insert it in the cylinder, screw cap 2 down tightly, which will cause 
all of the parts to assume their proper places. 

Testing Action. Before placing the spring on the indicator, it 
is well to test the indicator in order to determine whether or not it 
is in good condition. To do this, put the indicator together carefully, 
omitting the spring, oil the piston, and place the parts in the cylinder. 



32 STEAM ENGINE INDICATORS 

Then raise the pencil as high as it will go and release it. If it returns 
to the bottom of its own accord, it is an assurance that everything 
is in alignment and that there is little friction in the moving parts. 
Adjustment. Length of Indicator Cord. Having carefully con- 
nected the parts of the indicator and attached it to the engine cylin- 
der in the proper manner, the next step is to adjust the length of the 
indicator cord, which should be as short as possible. If it is impos- 
sible to use a short cord, then a fine steel or brass wire should be 
used. The builders of indicators usually furnish a braided cord 
which has been well stretched and which gives good results. Some- 
times it is convenient to make a loop in the end of the cord, which 
is engaged by a small hook attached to the reducing motion. One 
method for adjusting the length of the cord is as follows: The hook 
A, Fig. 34, is attached to the indicator cord. The cord B from the 




Fig. 34. Device for Altering Length of Indicator Cord 

reducing motion which passes through the holes in the plate P, as 
shown, is adjusted in length by slacking it at the point B and slip- 
ping the plate along the cord. To avoid an accident, in the way of 
injuring the indicator or the reducing motion, it is best to deter- 
mine as nearly as possible the necessary length of the cord before 
hooking up to the reducing motion. To determine the length of 
the cord, take hold of the end of it, and the hook to which it is to be 
attached; holding them in their relative positions, follow the motion 
of the reducing lever, keeping the cord tight, thus pulling the drum 
from one stop to the other, observing if the string must be length- 
ened or shortened to insure the drum traveling the proper distance. 
Having determined the length of the cord, hook the two cords 
together and ascertain whether or not the indicator drum strikes 
the stop at either end of the stroke. 

Adjusting Card and Pencil. Having made the proper adjust- 
ment of the length of cord, put the indicator card on the paper drum, 
being sure that it is smooth and even, as otherwise the diagram will 



TEAM ENGINE INDICATORS 33 

not be a true representation of the pressure in the cylinder. Con- 
siderable practice is required before one can put cards on the drum 
smoothly and rapidly, but this is desirable. After having caught 
both ends of the card by the clips, bend the ends over so that they 
will not interfere with the pencil arm. 

Adjust the pencil stop so that the pencil can bear only very 
lightly on the paper when in the proper position. Always use a 
pencil with a smooth sharp point, so the lines obtained will be plain 
and fine. A fine pointed pencil produces less friction when in con- 
tact with the paper, which is desirable. The pencil may be easily 
sharpened by using a small piece of sandpaper or a file. 

TAKING CARDS 

Everything being in readiness, attach the cord to the reducing 
motion and, with the pencil off of the card, open the cock and let 
steam pass into the cylinder of the indicator for a few strokes to 
warm it up; then put the pencil in contact with the paper for one 
revolution, after which turn the cock so that no steam is in the cylin- 
der. Again hold the pencil point in contact with the paper, thus 
getting the atmosphere line. The atmosphere line should always 
be taken last, in order that there is assurance that all the parts of 
the indicator are of the same temperature. 

Condition of Indicator. It is important to know that the indi- 
cator is working properly at the beginning of the test, so after tak- 
ing the first card, the indicator may be tried to see if it is working 
correctly. Open the cock and let the piston make a few strokes, 
close the cock, place the pencil in contact with the paper, at the same 
time turning the drum by hand; if the pencil retraces the original 
atmosphere line, or if after a slight pressure up or down on the piston, 
the pencil returns to the atmosphere line, it is evidence of its being 
in good condition. If the indicator fails to do the above, the pencil 
movement is not free in its joints, there is lost motion, or the piston 
does not move freely in the cylinder of the indicator. 

A card should be taken from both ends of the cylinder, for there 
can be no positive assurance that the same conditions exist in both 
ends. In fact, oftentimes there is quite a difference between the 
cards for the head end (h. e.) and the crank end (c. e.), due to inac- 
curacies of the valve motion and other defects. 



34 



STEAM ENGINE INDICATORS 



Sample Indicator Card. The cards are made of a good grade 
of white paper, one side being finished smooth. It is on the smooth 
side that the diagram is made. Manufacturers frequently furnish 
blank indicator cards having printed on the back of each a set of 
blank spaces to fill out, which is convenient for filing for the purpose 
of preserving important data. All of the blank spaces may not be 
filled out each time, but the more important points should never be 
neglected. The following form has been used by different success- 
ful engineers: 



Date Name of 

operator 

Time Owner of engine 

Diam. of cylinder Kind of work 

done by 
Length stroke engine 



Builder of 

... engine 

••• Kind of 

valve motion . . 
Kind of 

steam valves. . 
"'* Kind of 

R.P.M Remarks exhaust valves 

Kind of 
Speed of piston 



Diam. piston rod 

Barometer 
Area steam port reading. . 



Area exhaust port . 
Piston clearance. . 
Port clearance. . . . 
Boiler pressure 
Initial pressure. . . 
M.E.P 



condensers . 
Kind of 

heater 

Kind of 

boiler 

Kind of 

fuel 



Temperature 

feed water. 
Temperature 

hot well . . . 
Water per 

hour 

Coal per 

hour 



Indicator Card Analysis. Meaning of Lines of Diagram. Fig. 
35 illustrates a typical indicator card with all reference lines and 
events of the stroke designated by letters. It is to be remembered 
that the indicator card shows the relation between piston position 
and pressures in the cylinder. So on the diagram, all vertical lines 
or ordinates represent pressures and all the horizontal lines or abscis- 
sas represent distances. Bearing this distinction in mind, the pres- 
sure in the engine cylinder at any piston position, measured along 
the horizontal line, can be obtained by measuring the vertical height 
of the diagram at the point representing the piston position. There 



STEAM ENGINE INDICATORS 



35 



has been a set of lines placed on the indicator card shown, known as 
reference lines. These lines are Y, Y K, and OX. The other 
lines DE, EF, F G, GH, etc., are drawn by the indicator, as is 
also the atmosphere line AB, and it is the result of one indication 
from one side of the engine piston, say the head end side. The 
diagram for the crank end would be quite similar, but reversed in 
position on the paper. 

The reference lines are the atmosphere line A B, boiler pressure 
line Y K, clearance line Y, and the vacuum or absolute pressure 
line X. The atmosphere line is drawn by the indicator, when both 
sides of the indicator piston are acted upon by the pressure of the 
atmosphere only. Since this line is used as a reference line in meas- 




Fig. 35. Typical Indicator Card 



uring all pressures, it should be carefully located. The vacuum or 
absolute pressure line OX, the zero line of pressure, is drawn by 
hand below and parallel to the atmosphere line a distance by scale 
equal to the barometric pressure, which at sea level is 14.7 pounds 
per square inch. 

The line of boiler pressure J K is drawn above and parallel to 
the atmosphere line a distance by scale equal to the boiler pressure 
by gauge. The distance between the boiler pressure line and the 
line D E represents the loss in pressure that occurs while the steam 
is passing from the boiler to the cylinder of the engine. 

The clearance line Y is drawn at right angles to the atmos- 
phere line and at a distance from the extremity of the diagram equal 
to the per cent of clearance of the engine multiplied by the horizon- 
tal length of the diagram. That is, if the clearance is 2 per cent of 



36 STEAM ENGINE INDICATORS 

the piston displacement and the length of the card is 4 inches, then 
the distance between the extremities of the card and the clearance 
line would be 2 per cent of 4, which is .08 of an inch. By the term 
"clearance" is meant the volumetric space between the piston and 
the bottom of the valve when the engine is on dead center and this 
may differ in amount at each end of the cylinder. Obviously this 
space must be filled with steam from the boiler at the initial pres- 
sure at every stroke of the piston. It is, therefore, desirable to have 
as small a per cent of clearance as is consistent with good design, in 
order to eliminate the loss of live steam. For slow-speed engines 
the clearance space needs to be small — about 2 to 5 per cent, 
whereas for high-speed engines, it may run as high as 10 per cent. 
Measurement of Clearance. If the per cent of clearance is not 
given by the builders, it becomes necessary to measure it if any 
scientific study is to be made of the performance of the machine. 
Professor John E. Sweet gives a very simple plan for obtaining the 
per cent of clearance as follows: 

See that the piston and valves are made tight, and the valves discon- 
nected. Arrange to fill the clearance space with water through the indicator 
holes, or through holes drilled for the purpose. Turn the engine on dead 
center; make marks on the crosshead and guides; weigh a pail of water, and 
from it fill the clearance space. ^Yeigh the remaining water so as to deter- 
mine how much is used. Then weigh out exactly the same amount of water 
(as is used) , turn the engine off the center, pour in the second charge of water, 
and turn the engine back till the water comes to the same point that it did in 
the first instance. Make another mark on the crosshead and guide, and the 
distance between these marks is exactly what you really wish to know; that is, 
it is just what piston travel equals the clearance. If it takes one pound of 
water to fill this space and to admit another pound, the piston must be moved 
1 inch; then the clearance bears the same relation to the capacity of the cylin- 
der as 1 inch bears to the stroke of the piston. Thus, under these circum- 
stances, in an engine of 10-inch stroke, it would be said to have 10 per cent 
clearance. 

The above method would be correct when the engine is new, 
the piston and valves being tight and there being no leaks while the 
trial is being made; on the other hand, if the engine is old and the 
piston and valves are worn, it would only give approximate results. 
In such a case, it would be advisable to ascertain the per cent of 
clearance from the card by the following simple method. In Fig. 
35 draw the straight line L N from a point L on the vacuum line in 



STEAM ENGINE INDICATORS 37 

such direction that it will cut the compression curve at two points, 
as H and M. Now with a pair of dividers, set one leg on the point 
L and adjust the other to the point H. With the dividers thus set, 
place one leg in the point M, where L N cuts the compression curve, 
then sweep an arc cutting L N at P. Erect a line perpendicular to 
the vacuum line and passing through the point P, which establishes 
the clearance line AP Y. 

Events of the Cycle. While the steam engine is making one com- 
plete revolution, four separate and distinct events occur, namely, 
the admission, cut-off, release, and compression of steam. The 
point in the stroke where these events occur can be very accurately 
determined from the indicator diagram. Corresponding to the 
above events, there are six distinct lines on the card, namely, admis- 
sion, steam, expansion, release, compression, and back pressure. 
By properly analyzing the diagram in Fig. 35, these events and lines 
are easily determined. 

The admission line C D shows the rise of pressure in the cylinder due to 
the opening of the steam valve permitting steam to enter the cylinder. The 
point C indicates the point in the stroke at which the admission of steam took 
place. 

Steam line DE is drawn while the valve remains open and steam is 
being admitted to the cylinder. 

At the point E, the valve closes the steam port, thus cutting off the 
supply of steam to the cylinder, hence E is the point of cut-off (c. o.). 

As the motion continues, the volume back of the piston is increased and 
the pressure drops, due to the expansion of the steam, giving the expansion 
curve EF. 

The point of release occurs at F, where the valve uncovers the exhaust 
port, permitting the steam to escape from the cylinder. 

As soon as the point of release F is reached, the pressure begins to drop 
and by the time the point G has been reached on the return stroke practically 
all of the steam has been exhausted, hence F G is called the release line. 

The back pressure line GH indicates the amount of pressure against 
the engine piston while making the return stroke. On noncondensing engines, 
it is either coincident or above the atmosphere line. On condensing engines, 
it is found below the atmosphere line a distance depending upon the amount of 
vacuum being maintained. 

At the point H the exhaust port is closed and the steam remaining in the 
cylinder is compressed. The line HC indicates the rise of pressure due to 
the compression. 

The events of the stroke — as cut-off at E, release at F, com- 
pression at H, and admission at C — are not always easily located on 



38 



STEAM ENGINE INDICATORS 



the card, as there may not be a distinct change in the curve where 
these different events occur. The engineer must use his best judg- 
ment in locating these points. As an aid in making a decision, if 
one carefully inspects the diagram, it will be noted that when cut-off 
takes place at E, for instance, the steam line is concave and the 
expansion line convex downward, hence where these two opposite 
curves join must be the point at which cut-off occurs. Even this 
suggestion does not always hold but it will serve as an aid. 

The events are usually expressed in per cents of the stroke. To 
obtain these per cents, proceed as follows: Locate the events as 
described above. Consider the point of cut-off as located on the 
card shown in Fig. 36. From this point, draw a line perpendicular 
to the atmosphere line, cutting it at the point G. Measure the length 




- / F 3 4^56 7 89 

Fig. 36. Indicator Card Laid Out for Determining 
the M. E. P. 



of the card between the ordinates C D and F L; measure also the 

distance C G. The per cent of stroke at which cut-off occurs would 

C C 
be — — XlOO. C G equals 1.62 inches; CL equals 3.72 inches. 

C Li 

Therefore 

1.62 
Per cent of cut-off = -— XlOO = 43.5 per cent 

O. I — 

To find the per cent of release, admission, or compression, pro- 
ceed in the same manner as for cut-off, always measuring the dis- 
tances from the admission end of the card. 

, Pressures. In discussing an indicator card, five different pres- 
sures are frequently considered, namely, initial, terminal, and back 
pressure, pressure at the events, and the mean effective pressure 



STEAM ENGINE INDICATORS 39 

The initial pressure is the pressure in the cylinder at or near 
the beginning of the stroke. It would be measured on a perpendic- 
ular line from the atmosphere line to the steam line at D in Fig. 35. 

The terminal pressure is the pressure measured above the vacuum 
line at the end of the stroke. It is the pressure that would have 
been acting against the piston at the end of the stroke if the steam 
had not been released earlier. It is measured by extending the 
expansion curve until it cuts a perpendicular at the end of the card 
at V, Fig. 35. V X measured to scale gives the amount of the termi- 
nal pressure. 

Back pressure, which is the pressure the piston works against 
on the return stroke, is the distance between the atmosphere line 
AB and the back pressure line GH, Fig. 35. 

The pressure at the events is obtained by scaling a perpendicular 
line drawn from the points in question to the atmosphere line. 

The mean effective pressure, usually written m.e.p., is the net 
average pressure that acts on the piston throughout the entire stroke. 
It is evident from examination of Fig. 35 that the m.e.p. is the 
average height of the card multiplied by the scale of the spring used 
in the indicator. 

There are two general ways of obtaining the m.e.p., viz, by the 
use of a planimeter and by the ordinate method. 

In Fig. 36, CL is the atmosphere line and CD and FL are 
perpendiculars drawn at the end of the card. Divide CL into ten 
equal divisions, as 1, 2, 3, etc., and midway between C-l, 1-2, etc., 
draw the lines shown. Measure these lines and mark their lengths 
on them. When this is done, obtain the sum of all of these lengths, 
which, in this case, is 15.18 inches; and 15.18 inches divided by 
10 gives 1.518 inches, the average height of the card. If the scale, 
of the spring used was 40 pounds, then 1.518 inches multiplied by 
40 gives 60.72 pounds as the m.e.p. 

The ordinate method for finding the area of a card, then, is to di- 
vide the atmosphere line into ten or more equal divisions and, half way 
between these divisions, erect ordinates and divide the sum of all the or- 
dinates by the number of lines and multiply by the scale of the spring. 

The average m.e.p. for one revolution would be the average of 
the two mean effective pressures as determined from cards taken 
from both the head and the crank end of the cylinder. 



40 



STEAM ENGINE INDICATORS 



The number of divisions into which the card is divided could 
have been twenty as well as ten or any other number, but as ten or 
twenty makes the computations simple, they are usually taken. 

More accurate results will be ob- 
tained if a greater number of 
divisions are made, other things 
being equal. 

Determinakm of Mean Effective 
Pressure by Planimeter. A more 
accurate way of obtaining the 
m.e.p. is by using an instrument 
called the planimeter, of which 
there are several types in common 
use. The Anisler polar planim- 
eter is one of the most simple, 
and as shown in Fig. 37 is about 
one-half the size of the instru- 
ment. It consists of two arms 
free to move about a pivot and a 
roller graduated in square inches 
and tenths of square inches. A vernier is placed with the roller so 
the areas may be read in hundredths of a square inch. The point A 
is kept stationary and the tracer B is moved once around the outline 
of the diagram. The area in square inches of the diagram is read 
from the roller C and the vernier E. 




Fig. 37. 



Measuring Area of Diagram by 
Means of Planimeter 



INSTRUCTIONS FOR USE OF PLANIMETER 

The diagram should be fastened to some flat unglazed surface, such as a 
drawing board, by means of thumb tacks, springs, or pins. The point A is 
pressed into the paper so that it will hold in place: B is set at any point in the 
outline of the diagram: and the roller is set at zero. Follow the outline of 
the diagram carefully in the direction of the hands of a watch, as indicated by 
the arrows in Fig. 37, until the tracer has moved completely around the dia- 
gram. The result is then read to hundredths of an inch from the roller. Sup- 
pose, after tracing over the outline, we find that the largest figure that has 
passed the zero of the vernier is 3; the number of graduations (tenths) that 
have passed the zero are 5; and the graduation (hundredths) on the vernier 
that exactly coincides with a graduation on the roller is 9. Then the area is 
3.59 square inches. 

Often at the start the roller is not adjusted so that the zeros coincide, 
but the reading is taken and subtracted from the final reading. Thus, if the 



STEAM ENGINE INDICATORS 41 

first reading is 4.63, and the second 7.31, the area is 7.31 — 4.63 or, 2.68 square 
inches. In case the second reading is less than the first, add 10 to the second 
reading, then subtract. 

Briefly stated, to find the m.e.p. of a diagram, first ascertain the area 
of the card by use of the planimeter, multiply the area obtained by the scale 
of the spring, and divide the product by the length of the card in inches. This 
may be demonstrated in the following manner: If in a rectangle A equals 
area in square inches, L equals length in inches, and H equals height in inches, 
then 

A=LH 
If P is the average pressure, then 

average pressure 
scale of spring 
or 

scale 

Substituting this value in the equation A —h H, we have 

LXP 
scale 
or 

yl X scale 

The planimeter is a very valuable instrument to an engineer 
in taking indicator cards, the results obtained being very accurate. 
Ten or twelve diagrams can be measured by this instrument in the 
same time that is necessary to measure a single card by the method 
of ordinates . 

It is well to run over the area two or three times and take an 
average, as the tracing of the diagram can not be absolutely correct 
at any time. 

PHYSICAL THEORY 

In the study of any subject, there are always a number of technical 
terms that need to be clearly understood before a proper understand- 
ing of the subject is obtained. This is especially true with the 
steam engine, and the indicator, and with steam which is the motive 
force of each. It is, therefore, thought best to treat these terms and 
a study of the properties of steam together, at this point, in order 
to be able to go more deeply into the study of the uses of the indicator. 



42 STEAM ENGINE INDICATORS 

Pressure. Pressure is the force tending to compress a body and 
is usually expressed either in pounds per square inch, pounds per 
square foot, inches of mercury, or inches of water. 

Boiler Pressure. Boiler pressure is the pressure of steam in 
pounds per square inch above atmosphere as indicated by a steam 
gauge. 

Absolute Pressure. Absolute pressure is the pressure as obtained 
from absolute vacuum. At the sea level, the pressure of the atmos- 
phere is usually taken as 14.7 pounds per square inch; hence, if at 
sea level, a steam gauge reads one hundred pounds, the absolute 
pressure would be 100+14.7 or 114.7 pounds per square inch ab- 
solute. 

Atmospheric Pressure. Atmospheric pressure is the pressure 
the atmosphere exerts upon a body. It is usually measured in inches 
of mercury, as obtained from a barometer, hence it is sometimes 
spoken of as barometer pressure. Since the weight of 1 cubic inch 
of mercury is known to be .49 pounds, the reading of a barometer 
can be easily converted into pounds per square inch. If a barom- 
eter reads 28 inches of mercury, the pressure of the atmosphere 
expressed in pounds per square inch would be 28 X. 49 = 13.72. 

Pressure below atmosphere is also given in inches of mercury 
and sometimes in inches of water. If it is desired to obtain the 
absolute pressure in pounds per square inch, reduce the reading in 
inches of mercury to pounds per square inch and subtract this 
amount from the atmospheric pressure expressed in pounds per 
square inch. 

Example. A vacuum gauge on a steam engine condenser reads 26.1 
inches of mercury. The barometer stands at 29.12 inches of mercury. What 
is the absolute pressure in the condenser in pounds per square inch? 

Solution. 29.12 inches of mercury is equivalent to 29.12 X .49 = 14.268 
pounds per square inch and 26.1 inches is equivalent to 26.1 X. 49 = 12.789 
pounds per square inch. Therefore, the absolute pressure in pounds per 
square inch in the condenser is 14.268—12.789 = 1.479. 

Work. The unit of work is called a foot pound and it is equal 
in amount to the energy required to lift one pound one foot high. 
It is to be noted that it is the product of a force times the distance 
through which it acts. If a weight of 50 pounds be raised 7 feet 
high, then 50X7 or 350 foot pounds of work would be expended. 



STEAM ENGINE INDICATORS 43 

Heat. Temperature. By temperature is meant simply the 
thermal condition of a body with reference to its capability of trans- 
ferring heat to other bodies. If two bodies are placed in contact 
and the first gives more heat to the second than it receives, we say 
that No. 1 is hotter than No. 2. If the first receives more heat 
than it gives, No. 2 is hotter than No. 1. If both bodies give and 
receive the same amount of heat, they are of the same temperature. 

x\ccording to our theory, it is evident that temperature depends 
upon the energy of molecular vibration. If the temperature rises, 
it means that the molecular vibration, and consequently the energy 
increases; if the temperature falls, the energy of molecular vibration 
decreases. Evidently, a point must finally be reached when this 
energy of vibration is zero and the molecules are at rest. At this 
temperature, there is no beat and we call it the absolute zero. 
This zero is evidently mar below the zero of the ordinary scale. 

Thermometers, In ord_ to determine just how hot a body is, 
we must compare its temperature with that of some substance whose 
degree of heat we know. As it would be impossible to keep several 
bodies at different degrees of heat for comparison, we must resort to 
some other means. A simple method is to use some substance whose 
volume changes a definite amount for a definite change in tempera- 
ture and always has the same volume for the same temperature. 
Mercury and alcohol are suitable substances and may be placed in 
a glass bulb, to which is connected a glass tube of small bore. All 
the air is drawn out of the tube, and the end is sealed so that the 
thermometric substance can expand or contract in a vacuum. The 
tube having been sealed, the bulb is placed in melting ice and the 
height of the mercury in the tube noted. It is then placed in steam 
(or boiling water) at atmospheric pressure and the height of the 
column again noted. On the Fahrenheit scale, the melting point 
is called 32 degrees and the boiling point 212 degrees, and the inter- 
vening space is divided into 180 equal parts. In the centigrade 
scale, the melting point is called zero degree and the boiling point 
100 degrees; there are 100 equal intervals between them. Thus we 
see that 180°F. = 100°C, or 1°C. = 1.8°F. 

Example. What is the temperature of 50°C. on the Fahrenheit scale? 

50°C. =50X1.8 = 90°F. above the melting point 
= 90+32 = 122°F. 



44 STEAM ENGINE INDICATORS 

. In order to compare temperatures, we place the thermometer 
in contact with the substance whose degree of heat we wish to know 
and then observe the height of the liquid column in the thermometer. 
The height of this column depends upon the expansion of the ther- 
mometry substance and indicates the intensity of heat, or the tem- 
perature as we commonly call it. We use a thermometer to measure 
the intensity of heat, but not the quantity of heat. 

Unit of Heat Quantity. For measuring the intensity of heat, 
the degree is the unit; for measuring the quantity of heat, we have 
another unit, which is the amount of heat necessary to raise one 
pound of water from 61° F. to 62° F. This is called the British 
thermal unit (B.T.U.). To raise one pound of water from 60° F. 
to 62° F., or to raise two pounds from 60° F. to 61° F., will require 
2 B.T.U. 

One B.T.U. is equivalent to 778 foot pounds of work. If one 
pound of coal liberates 12,000 B.T.U. when burned, it is capable of 
producing 12,000x778 = 9,336,000 foot pounds of work. 

The above value of the B.T.U. in foot pounds of work is known 
as the mechanical equivalent of heat, that is, 778 foot pounds. 

Horsepower. Horsepower is the arbitrary standard used for 
measuring the power of a steam engine. It is said to have been 
originally established by James Watt from experiments conducted 
with dray horses on the streets of London. Its value is, however, 
considerably above that of the ordinary horse. It is defined as being 
equal to lifting 33,000 pounds one foot high in one minute of time. 
It will be noted that the horsepower takes into account the follow- 
ing factors: force, distance, and time. This being true, it is desirable 
to have an expression in the form of an equation to express the 
horsepower of an engine. The common formula for steam engines is 

PLAN 

h.p. = 

33000 

in which P equals the mean effective pressure in pounds per square 
inch; L equals length of stroke in feet; A equals area of cylinder in 
square inches; N equals number of revolutions per minute (r.p.m.); 
and 33,000 equals equivalent foot pounds of work per minute in one 
horsepower. 

Analyzing the equation, it is found that it conforms to the defini- 



STEAM ENGINE INDICATORS 45 

tion of work previously given. For instance, A, the area of the 
piston in square inches, times P, the mean effective pressure in 
pounds per square inch, is equal to the total force on the piston which 
acts through the stroke a distance of L feet. Hence, the expression 
PL A gives the foot pounds of work done during one stroke. In 
the definition of horsepower, it was noted that the time element was 
considered, so we have (PLA)XN divided by 33,000, fulfilling 
the definition of a horsepower, since N involves the element of time. 
Indicated Horsepower. The indicated horsepower (i.h.p.) is 
the computed horsepower of an engine as obtained by using an indi- 
cator diagram taken from the engine cylinder. From this diagram 
is determined P, the mean effective pressure, which is substituted in 
the equation just given. 

Example. Given an engine having a cylinder 10 inches in diameter and 
a stroke 16 inches in length, running at 180 r.p.m. The mean effective pres- 
sure on the piston as obtained from the indicator card is 75 pounds on both 
the head and the crank end of the cylinders. Required the horsepower of the 
engine. 

Solution. In this example P equals 75 pounds; L equals 16-f-12, or 
1.33 feet; A equals -R' 1 equals 3.1416 X5 2 or 78.54 Square inches; and N 
equals 180 r.p.m. 

Substituting these values in the formula 
_ PLAN 
' P ' ~ 33000 
we have 

75X1.33X78.54X180 _■ 

h.p. = =42.75 

1 33000 

This is for one end of the cylinder only. For both ends, we get the total 

h.p. =42.75X2 =85.5 (apr~oximately) 

Engine Constant. It is eviden from the foregoing problem 
that, for a given engine, some factors iix the h.p. formula remain con- 
stant. These constants are: the area 3f the piston, the length of the 
stroke, and the abstract number 33,000. It is convenient when 
making a large number of computations to determine what is known 
as the engine constant, a factor which saves considerable time and 
reduces the chances of error. Since the area of the piston on the 
crank end is smaller than that on the head end by an amount equal 
to the area of the piston rod, the engine constant for the crank end 
is always slightly smaller than for the head end. 



46 STEAM ENGINE INDICATORS 

Example. Find the constant for both h.e. and c.e. of the engine in the 
preceding problem, whose piston rod was If inches in diameter. The engine 
constant for the head end is 

LA 
Che - "33000 

in which L equals 16-^12, or 1.33 feet, A equals 3.1416 Xo 2 or 78.54 square 

inches. 

1.33X78.54 

Ch.e. = — — — — - =.00316 

33000 

For the crank end, the area of piston is reduced by the area of the If -inch 
piston rod, which area is equal to 2.40 square inches. The effective area for 
the crank end is, therefore, 78.54 — 2.40, or 76.14 square inches. 

1.33X76.14 

C ce = =.003068 

33000 

Having obtained the engine constant, in order to obtain the indicated horse- 
power (i.h.p.) it is only necessary to multiply the m.e.p. on the piston for each 
end of the cylinder by the engine constant for that end and by the number of 
revolutions. 

In Table II, the approximate i.h.p. of an engine is easily found 
by multiplying the constant, corresponding to the diameter of the 
piston, by the piston speed and by the m.e.p. Or, in other words, 
the constants in the table equal the h.p. for an engine with a given 
diameter of piston having a piston speed of one foot per minute 
and a m.e.p. of one pound. The piston speed of any engine is equal 
to the length of stroke in feet multiplied by twice the number of 
revolutions. For instance, in the 10- by 16-inch engine already 
referred to, the piston speed in feet per minute would be 1.35 feetX 
180X2 = 478.8 feet per minute. 

If the diameter of the piston is an even number, the constant 
is found in the second column; if it contains a fraction, the constant 
is found by following the column horizontally until the required 
fraction is reached. The constant multiplied by the piston speed 
in feet per minute and by the m.e.p. in pounds per square inch gives 
the i.h.p. approximately. 

Example. An engine runs at 75 r.p.m. and the stroke is 4 feet. If the 
m.e.p. is 48 and the piston is 27| inches in diameter, determine the i.h.p. 

Solution. From Table II, the constant for a piston 27f inches in diam- 
eter is .0178355. The piston speed is 4X75X2=600 feet per minute. Then 

i.h.p. = .0178355 X600 X48 
= 513.66 (approx.) 



STEAM ENGINE INDICATORS 



47 



TABLE II 
Engine Constants 



f - 
Diameter 


Even 
Inches 


+*" 


+ \" 


+1" 


+ §" 


+ i" 


+1" 


+1" 


of 


or 


or 


or 


or 


or 


or 


or 


Cylinder 


.125 


.25 


.375 


.5 


.625 


.75 


.875 


1 


. 0000238 


.0000301 


. 0000372 


. 0000450 


.0000535 


. 0000628 


.0000729 


. 0000837 


2 


. 0000952 


.0001074 


.0001205 


.0001342 


.0001487 


.0001640 


.0001800 


.0001967 


3 


.0002142 


.0002324 


.0002514 


.0002711 


.0002915 


.0003127 


.0003347 


.0003574 


4 


.0003808 


. 0004050 


. 0004299 


. 0004554 


.0004819 


.0005091 


.0005370 


. 0005656 


5 


. 0005950 


.0006251 


. 0006560 


. 0006876 


.0007199 


.0007530 


.0007869 


.0008215 


6 


. 0008568 


. 0008929 


.0009297 


.0009672 


.0010055 


.0010445 


.0010844 


.0011249 


7 


.0011662 


.0012082 


.0012510 


.0012944 


.0013387 


.0013837 


.0014295 


.0014759 


8 


.0015232 


.0015711 


.0016198 


.0016693 


.0017195 


. 0017705 


.0018222 


.0018746 


9 


.0019278 


.0019817 


.0020363 


.0020916 


.0021479 


.0022048 


.0022625 


. 0023209 


10 


. 0023800 


.0024398 


. 0025004 


.0025618 


.0026239 


.0026867 


. 0027502 


.0028147 


11 


.0028798 


. 0029456 


.0030121 


. 0030794 


.0031475 


.0032163 


.0032859 


.0033561 


12 


. 0034272 


. 0034990 


.0035714 


. 0036447 


.0037187 


. 0037934 


. 0038690 


.0039452 


13 


.0040222 


. 0040999 


. 0041783 


.0042576 


.0043375 


.0044182 


. 0044997 


.0045819 


14 


. 0046648 


0047484 


.0048328 


.0049181 


.0050039 


.0050906 


.0051780 


.0052661 


15 


.0053550 


0054446 


. 0055349 


0056261 


.0057179 


.0058105 


.0059039 


. 0059979 


16 


. 0060928 


.0061884 


0062847 


.0063817 


.0064795 


.0065780 


. 0066774 


.0067774 


17 


. 0068782 


. 0069797 


.0070819 


0071850 


.0072887 


.0073932 


. 0074985 


0076044 


18 


.0077112 


0078187 


0079268 


. 0080360 


.0081452 


.0082560 


.0083672 


.0084791 


19 


.0085918 


0087052 


. 0088193 


0089343 


.0090499 


.0091663 


0092835 


.0094013 


20 


. 0095200 


0096393 


.0097594 


. 0098803 


.0100019 


.0101243 


. 0102474 


.0103712 


21 


0104958 


0106211 


0107472 


.0108739 


.0110015 


.0111299 


.0112589 


0113886 


22 


.0115192 


.0116505 


.0117825 


.0119152 


.0120487 


.0121830 


.0123179 


.0124537 


23 


.0125902 


0127274 


.0128654 


. 0130040 


.0131435 


.0132837 


.0134247 


.0135664 


24 


.0137088 


.0138519 


.0139959 


.0141405 


.0142859 


.0144321 


.0145789 


.0147266 


25 


.0148750 


.0150241 


0151739 


.0153246 


.0154759 


.0156280 


.0157809 


.0159345 


26 


.0160888 


.0162439 


.0163997 


.0165563 


.0167135 


.0168716 


.0170304 


.0171899 


27 


.0173502 


0175112 


.0176729 


.0178355 


.0179988 


.0181627 


.018*3275 


.0184929 


28 


.0186592 


.0188262 


.0189939 


.0191624 


.0193316 


.0195015 


.0196722 


.0198436 


29 


.0200158 


.0201887 


. 0203624 


.0205368 


.0207119 


.0208879 


.0210645 


.0212418 


30 


.0214200 


.0215988 


.0217785 


.0219588 


.0221399 


.0223218 


. 0225044 


.0226877 


31 


.0228718 


.0230566 


.0232422 


.0234285 


.0236155 


.0238033 


.0239919 


.0241812 


32 


.0243712 


.0245619 


.0247535 


. 0249457 


.0251387 


.0253325 


.0255269 


.0257222 


33 


0259182 


.0261149 


.0263124 


.0265106 


.0267095 


.0269092 


.0271097 


.0273109 


34 


.0275128 


.0277155 


.0279189 


.0281231 


. 0283279 


.0285336 


.0287399 


.0289471 


35 


.0291550 


.0293636 


. 0295729 


.0297831 


.0299939 


.0302056 


.0304179 


.0306309 


36 


. 0308448 


.0310594 


.0312747 


.0314908 


.0317075 


.0319251 


.0321434 


.0323624 


37 


.0325822 


. 0328027 


.0330239 


.0332460 


.0334687 


.0336922 


.0339165 


.0341415 


38 


.0343672 


.0345937 


. 0348209 


.0350489 


.0352775 


.0355070 


.0357372 


.0359681 


39 


.0361998 


. 0364322 


.0366654 


. 0368993 


.0371339 


.0373694 


.0376055 


.0378424 


40 


.0380800 


.0383184 


.0385575 


.0387973 


.0390379 


.0392793 


.0395214 


.0397642 


41 


.0400078 


.0402521 


. 0404972 


.0407430 


. 0409895 


.0412368 


.0414849 


.0417337 


42 


.0419832 


.0422335 


. 0424845 


. 0427362 


.0429887 


.0432420 


.0434959 


.0437507 


43 


. 0440062 


. 0442624 


.0445194 


.0447771 


. 0450355 


. 0452947 


.0455547 


.0158154 


44 


.0460768 


. 0463389 


.0466019 


. 0468655 


.0471299 


.0473951 


.0476609 


.0479276 


45 


.0481950 


.0484631 


.0487320 


.0490016 


.0492719 


. 0495430 


.0498149 


.0508875 


46 


. 0503608 


. 0506349 


. 0509097 


.0511853 


.0514615 


.0517386 


.0520164 


.0522949 


47 


.0525742 


.0528542 


.0531349 


.0534165 


.0536988 


.0539818 


.0542655 


.0545499 


48 


.0548352 


.0551212 


.0554079 


. 0556953 


.0559835 


. 0562725 


.0565622 


.0568526 


49 


.0571438 


.0574357 


.0577284 


.0580218 


.0583159 


.0586109 


. 0589065 


.0592029 


50 


. 0595000 


.0597979 


.0600965 


.0603959 


.0606959 


. 0609969 


.0612984 


.0616007 


51 


.0619038 


. 0622076 


.0625122 


.0628175 


.0632235 


.0634304 


.0637379 


.0640462 


52 


.0643552 


.0646649 


.0649753 


.0652867 


. 0655987 


.0659115 


.0662250 


. 0665392 


53 


. 0668542 


.0671699 


. 0674864 


. 0678036 


.0681215 


. 0684402 


.0687597 


.0690799 


54 


. 0694008 


.0697225 


.0700449 


. 0703681 


. 0705293 


.0710166 


.0713419 


.0716681 


55 


.0719950 


.0724226 


.0726510 


.0729801 


.0733099 


. 0736406 


.0739719 


. 0743039 


56 


.0746368 


.0749704 


. 0753047 


.0756398 


.0759755 


.0763120 


. 0766494 


.0769874 


57 


.0773262 


.0776657 


.0780060 


.0783476 


.0786887 


.0790312 


. 0793745 


.0797185 


58 


.0800632 


.0804087 


.0807549 


.0811019 


.0814495 


.0817980 


.0821472 


.0824971 


59 


.0828478 


.0831992 


.0835514 


.0839043 


.0842579 


.0846123 


.0849675 


.0853234 


60 


. 0856800 


.0860374 1.0863955 


.0867543 


.0871139 


.0874743 


.0878354 


.0881973 



The result is only approximately correct on account of the area 
of the piston rod not being deducted from the area of the piston 
on the crank end. It is sufficiently accurate, however, for prac- 
tical purposes. 



48 STEAM ENGINE INDICATORS 

Brake Horsepower. All of the i.h.p. is not available for useful 
work as the internal friction of the engine absorbs some of the energy, 
so the net horsepower is the i.h.p. less the h.p. consumed by the engine 
in overcoming internal resistances. This net horsepower is usually 
spoken of as the brake horsepower (b.h.p.) and it is obtained by the 
use of some form of brake. 

Mechanical Efficiency. The mechanical efficiency of an engine 

is the ratio between the b.h.p. and the i.h.p. Expressed in per. cent, 

.„ b.h.p.XlOO T . . . , . ,. . 

it would be — ; • It is sometimes given as the engine friction 

i.h.p. 

in per cent, that is, the mechanical efficiency is expressed as ten or 
fifteen per cent engine friction, which is evidently 100 minus the 
mechanical efficiency. Under ordinary conditions, the engine fric- 
tion varies from about 6 to 10 per cent of the i.h.p. depending on the 
size and the construction of the engine. 

Piston Displacement. The piston displacement is the space 
in the cylinder swept through by the piston in its travel, expressed 
in cubic feet. The piston displacement for the c.e. will be less than 
for the h.e. by an amount equal to the area of the piston rod in square 
feet multiplied by the stroke in feet. In the 10- by 16-inch engine 
with If -inch piston rod, the piston displacement for head end is 

.7854X10 2 X16 
Piston displacement = — 

(Head End) 1728 

= .72722 cubic feet 

For the crank end, it would be .72722 less the cubic feet in the piston 
rod or 

Piston displacement = .72722 — ( I 

(Crank End) 1728 ' 

= .72722-. 02221 
= .70501 cubic feet 



PROPERTIES OF STEAM 

Saturated Vapor. The process of converting a liquid into a 
vapor is known as vaporization; the product thus formed is readily 
condensed and, therefore, does not follow the laws of perfect gases. 
A dry saturated vapor is one that has just enough heat in it to keep 



STEAM ENGINE INDICATORS 



49 



t oi l 



it in the form of a vapor; if we add more heat, it becomes super- 
heated. A superheated vapor may lose a part of its heat without 
condensation; a saturated vapor can not. When a saturated vapor 
loses a part of its heat, some of it will condense and we say that the 
vapor is wet. 

Steam is simply the vapor from water and we shall confine our 
discussion to this alone. Suppose we have a vertical cylinder, as 
shown in Fig. 38, fitted with a light piston free to move up and down, 
yet so constructed that it may be loaded at will. Suppose that 
there is one pound of water at a temperature of 32°F. in the bottom . 
of cylinder A, and that the piston rests upon its surface. Now, 
if heat is applied by means of a gas flame or fire, we shall notice the 
following effects: 

(1) The temperature of the water will gradually rise until it 
reaches the temperature at which steam is formed. This tempera- 
ture will depend upon the pressure, or the load on the piston. If 
the piston is very light, we may 

neglect its weight and consider 
that there is simply the atmos- 
pheric pressure of 14.7 pounds 
per square inch acting on the 
water surface, at which pressure, 
steam forms at 212° F. 

(2) Therefore, as soon as 
212 degrees is reached, steam will 
form and the piston will steadily 
rise (B, Fig. 38), but no matter how hot the fire may be, the tem- 
perature of both water and steam will remain at 212 degrees until 
all the water is evaporated (C, Fig. 38). 

We had one pound of water at 32 degrees and at 14.7 pounds 
absolute pressure, and found that steam formed at a temperature 
of 212 degrees and remained at that temperature. There were 
added 180.3 B.T.U., the heat of the liquid, to bring the water from 
32 degrees to the boiling point. To convert water at 212 degrees 
into steam at 212 degrees, there were added 969.7 B.T.U. more. 
This quantity, known as the latent heat, or heat of vaporization, 
makes the total heat 1150.0 B.T.U. If we should measure the 
volume carefully after all the water was evaporated, we should find 







^o 1 



Fig. 38. Engine Cylinder Containing Water 
and Steam 



50 STEAM ENGINE INDICATORS 

that there were exactly 26.78 cubic feet of dry saturated steam. 
At the start we had one pound of water and, therefore, we must 
have one pound of steam, for none could escape; hence one cubic 

foot will weigh — _ — , or 0.03734 pound, which is known as the 
26.78 

density of steam at 14.7 pounds absolute pressure and 212° F. 

Effect of Pressure on Boiling Point. Suppose now we place a 

weight of 85.3 pounds per square inch on the piston. The pressure 

is 85.3 plus 14.7 or 100 pounds per square inch absolute. We shall 

now find that no steam will form until a temperature of 327.86 

degrees is reached, and also we must add 887.6 B.T.U. Under this 

greater pressure the steam occupies a volume of only 4.432 cubic 

feet, or one cubic foot of it weighs _ — , or 0.2256 pound. ' 

4.432 

From the foregoing, it is obvious that there is a definite rela- 
tion between the pressure and the temperature, that is, the tempera- 
ture at which water will boil depends upon the pressure, and vice 
versa. 

Of course, it would be impossible to determine all these different 
quantities by actual experiment, and at all pressures varying from 
vacuum to the high pressures used in water-tube boilers; they can 
be computed. 

Steam Tables. We have already seen that any change in the 
temperature of saturated steam produces a change of pressure, and 
that every change of pressure corresponds to a certain change in 
temperature. There are several properties of saturated steam that 
depend upon the temperature and pressure; and the values of all 
these different properties when .arranged for all temperatures and 
pressures are called Steam Tables. The following are the principal 
items found in the tables: 

(1) Absolute pressure in pounds per square inch; it is equal to the gauge 
pressure plus the atmosphere pressure of 14.7 pounds or the pressure of atmos- 
phere as obtained from the barometer. 

(2) Temperature of steam, or of boiling water, at the corresponding pres- 
sure. 

(3) Heat of liquid; or the number of B.T.U. necessary to raise one pound 
of water from 32°F. to the boiling point corresponding to the given pressure. 

(4) Heat of vaporization, or latent heat; that is, the number of B.T.U. 
necessary to change one pound of water, at the boiling point, into dry satur- 



STEAM ENGINE INDICATORS 51 

ated steam at the same temperature and pressure. It was noted that in heat- 
ing water from 32°F. to the boiling point under atmosphere pressure, the 
temperature rose from 32 degrees to 212 degrees, but as soon as 212 degrees 
was reached, the temperature remained constant until all the water was con- 
verted into steam at that pressure. While the process of changing the water 
at 212 degrees into steam at 212 degrees was going on, heat was being added 
but no change occurred in temperature. This phenomenon always occurs when 
the application of heat results in the change of state of a substance, either from 
a solid to a liquid, or from a liquid to a gaseous state. This apparent loss of 
heat is not real, but recurs whenever the transformation is reversed, that is 
when the steam is condensed. 

(5) Total heat of vaporization is the number of B.T.U. necessary to change 
one pound of water from 32°F. into steam at the given temperature or pres- 
sure, and represents the sum of the heat units of the liquid plus the heat units 
of vaporization or the latent heat. 

(6) Density of steam, which is the weight of one cubic foot of steam at 
the given temperature or pressure. 

(7) Specific volume, which is the volume occupied by one pound of steam. 

The specific volume is the reciprocal of the densitv,that is, it is equal to— : — 

density 

Steam tables from which the above items may be obtained are 
very useful to the engineer. Any one wishing to make a more 
detailed study of the steam tables and the properties of steam should 
procure "Steam and Entropy Tables" by Peabody, published by 
John Wiley & Sons. 

Kinds of Steam. If the process of adding heat to water and 
then to steam be continued, three kinds of steam are obtained depend- 
ing on the conditions, namely, saturated or dry steam, wet steam, 
and superheated steam. 

Saturated or Dry Steam. If just sufficient heat be added to the 
vessel A, Fig. 38, until the pound of water is completely converted 
into steam as shown in C, the result is a pound of saturated steam. 
The number of heat units that have been added equals the heat of 
the liquid q plus the latent heat r. Therefore Q, the number of 
B.T.U. added, in equation form becomes 

Q = q+r 

It is evident, therefore, that saturated steam contains all the heat 
of the liquid plus the heat of vaporization. 

Wet Steam. If instead of adding enough heat to the vessel C 
to completely evaporate the water, the operation be discontinued 
while there is yet some water remaining unevaporated, as is shown 



TABLE III 
Properties of Saturated Steam 



Total 

pressure 
in lbs. 


Tempera- 
ture in 
degrees 


Heat in 

liquid 

from 32° 


Heat of 
vaporiza- 
tion or 


Total heat 

in heat 
units from 


Density or 
weight of 
one cubic 


Volume 
of 1 pound 


Total 
pressure 
in lbs. 


per sq. in. 

above 

vacuum 

P 


Fahren- 
heit 
t 


in 

heat units 

Q 


latent 

heat in 

heat units 

r 


water at 
32° 
H 


foot in lbs. 
1 


in cubic 
feet 


per sq. in. 

above 

vacuum 

V 


1 


101.84 


69.8 


1034.7 


1104.5 


0.00300 


333.1 


1 


2 


126.15 


94.2 


1021.9 


1116.1 


. 00578 


173.1 


2 


3 


141.52 


109.6 


1012.2 


1121.8 


0.00845 


118.4 


3 


4 


153.00 


121.0 


1005 . 5 


1126.5 


0.01106 


90.4 


4 


5 


162.26 


130.3 


1000.0 


1130.3 


0.01364 


73.3 


5 


6 


170.07 


138.1 


995.5 


1133.6 


0.01616 


61.9 


6 


7 


176.84 


144.9 


991.4 


1136.3 


0.01866 


53.6 


7 


8 


182 . 86 


150.9 


987.8 


1138.7 


0.02116 


47.26 


8 


9 


188.27 


156.4 


984.5 


1140.9 


0.02362 


42.36 


9 


10 


193.21 


161.3 


981.4 


1142.7 


0.02606 


38.37 


10 


14.7 


212.00 


180.3 


969.7 


1150.0 


0.03734 


26.78 


14.7 


15 


213.03 


181.3 


969.1 


1150.4 


0.03805 


26.28 


15 


20 


227.95 


196.4 


959 . 4 


1155.8 


0.04978 


20.09 


20 


25 


240.07 


208.7 


951.4 


1160.1 


0.06140 


16.29 


25 


30 


250.34 


219.1 


944.4 


1163.5 


0.0728 


13.74 


30 


35 


259.29 


228.2 


938.2 


1166.4 


0.0842 


11.88 


35 


40 


267.26 


236.4 


932.6 


1169.0 


0.0953 


10.49 


40 


45 


274.46 


243.7 


927.5 


1171.2 


. 1065 


9.387 


45 


50 


281.03 


250.4 


922.8 


1173.2 


0.1176 


8.507 


50 


55 


287.09 


256.6 


918.4 


1175.0 


0.1286 


7.778 


55 


60 


292.74 


262.4 


914.3 


1176.7 


0.1395 


7.166 


60 


65 


298.00 


267.8 


910.4 


1178.2 


. 1504 


6.647 


65 


70 


302.96 


272.9 


906.6 


1179.5 


0.1613 


6.199 


70 


75 


307.64 


277.7 


903.1 


1180.8 


0.1722 


5.807 


75 


80 


312.08 


282.2 


899.8 


1182.0 


. 1829 


5.466 


80 


85 


316.30 


286.5 


896.6 


1183.1 


0.1938 


5.161 


85 


90 


320.32 


290.7 


893.5 


1184.2 


0.2047 


4.886 


90 


95 


324.16 


294.6 


890.5 


1185.1 


0.2153 


4.644 


95 


100 


327.86 


298.5 


887.6 


1186.1 


0.2256 


4.432 


100 


105 


331.42 


302.1 


884.8 


1186.9 


. 2362 


4.233 


105 


110 


334.83 


305.6 


882.1 


1187.7 


0.2471 


4.047 


110 


115 


338.14 


309.0 


879.5 


1188.5 


0.2580 


3.876 


115 


120' 


341.31 


312.3 


876.9 


1189.2 


0.2686 


3.723 


120 


125 


344 . 39 


315.5 


874.5 


1190.0 


. 2793 


3.581 


125 


130 


347.38 


318.6 


872.1 


1190.7 


0.2898 


3.451 


130 


140 


353.09 


324.4 


867.4 


1191.8 


0.3106 


3.220 


140 


150 


358 . 50 


330.0 


863.0 


1193.0 


0.3318 


3.014 


150 


160 


363 . 62 


335.3 


858.8 


1194.1 


0.3528 


2.834 


160 


170 


368.50 


340.4 


854.8 


1195.2 


0.3741 


2.673 


170 


180 


373.16 


345.2 


850.9 


1196.1 


0.3951 - 


2.531 


180 


190 


377.61 


349.8 


847.1 


1196.9 


0.4158 


2.405 


190 


200 


381.89 


354.3 


843.5 


1197.8 


0.4371 


2.288 


200 


210 


386.02 


358.6 


840.0 


1198.6 


0.4579 


2.184 


210 


220 


389.98 


362.7 


836.6 


1199.3 


0.4789 


2.088 


220 


230 


393.80 


366.6 


833.3 


1199.9 


0.4997 


2.001 


230 


240 


397.50 


370.5 


830.1 


1200.6 


0.521 


1.921 


240 


250 


401 . 10 


374.2 


826.9 


1201 . 1 


0.542 


1.845 


250 


260 


404.55 


377.8 


823.9 


1201.7 


0.563 


1.775 


260 


270 


407.90 


381.3 


820.9 


1202.2 


0.584 


1.711 


270 


280 


411.19 


384.8 


818.0 


1202.8 


0.605 


1.652 


280 


290 


414.35 


388.1 


815.2 


1203.3 


0.627 


1.595 


290 


300 


417.45 


391.3 


812.4 


1203.7 


0.649 


1.542 


300 


310 


420.45 


394.4 


809.7 


1204.1 


0.670 


1.492 


310 


320 


423.40 


397.5 


807.1 


1204.6 


0.692 


1.446 


320 



TABLE IV 
Properties of Saturated Steam 



1 

Tempera- 


Total 


Heat in 


Heat of 
vaporiza- 


Total heat 


Density or 


Volume 


Tempera- 


ture in 


pressure 


liquid in 


tion or 


in heat 


weight of 


of 1 pound 


ture in 


degrees 


above 


heat units 


latent 


units from 


one cubic 


in cubic 


degrees 


Fahren- 


vacuum 


from 32° 


heat in 


water at 


foot in lbs. 


feet 


Fahren- 


heit 
t 


V 


Q 


heat units 


32° 


1 


s 


heit 
t 


32 


0.0886 


0.0 


1071.7 


1071.7 


. 000302 


3308.0 


32 


60 


0.2561 


28.1 


1057.0 


1085.1 


0.000828 


1207.0 


60 


90 


. 6960 


58.1 


1041.2 


1099.3 


0.002131 


469.2 


90 


120 


1.689 


88.0 


1024.4 


1112.4 


0.004926 


203 . 


120 


140 


2.885 


108.0 


1013.1 


1121.1 


0.00814 


122.8 


140 


1.50 


3.715 


118.0 


1007.2 


1125.2 


0.01032 


96.9 


150 


160 


4 . 738 


128.0 


1001.4 


1129.4 


0.01296 


77.2 


160 


170 


5.990 


138.0 


995 . 5 


1133.5 


0.01613 


62.0 


170 


180 


7.510 


148.0 


989.5 


1137.5 


0.01993 


50.2 


180 


190 


9.339 


158 . 1 


983.4 


1141.5 


. 02444 


40.92 


190 


200 


11.528 


168.2 


977.2 


1145.4 


0.02974 


33.62 


200 


212 


14.698 


180.3 


969.7 


1150.0 


. 03734 


26.78 


212 


220 


17.188 


188.4 


964.6 


1153.0 


0.04321 


23 . 14 


220 


225 


18.914 


193 . 4 


961.4 


1154.8 


. 04726 


21 . 16 


225 


230 


20.780 


198.5 


958 . 1 


1156.6 


0.05160 


19.37 


230 


235 


22 . 790 


203.6 


954 . 8 


1158.4 


0.05630 


17.77 


235 


240 


24 . 970 


208.6 


951.4 


1160.0 


0.06130 


16.31 


240 


245 


27.310 


213.7 


948.1 


1161.8 


. 06660 


15.01 


245 


2.30 


29.82 


218.8 


944 . 7 


1163.5 


0.0724 


13.82 


250 


255 


32.53 


223.8 


941.2 


1165.0 0.0785 


12.73 


255 


260 


35.42 


229.0 


937.8 


1166.8 


0.0851 


11.75 


260 


265 


38.53 


234 . 


934 . 3 


1168.3 


0.0920 


10.87 


265 


270 


41.84 


239 . 1 


930 . 7 


1169.8 


0.0995 


10.05 


270 


275 


45.39 


244.2 


927.2 


1171.4 


0.1074 


9.309 


275 


280 


49.19 


249 . 4 


923.6 


1173.0 


0.1158 


8.639 


280 


285 


53 . 22 


254 . 5 


920.0 


1174.5 


0.1247 


8.021 


285 


290 


57.53 


259.6 


916.3 


1175.9 


0.1341 


7.454 


290 


295 


62.11 


264.7 


912.6 


1177.3 


0.1441 


6.937 


295 


300 


66.98 


269.8 


908 . 9 


1178.7 


0.1547 


6.462 


300 


305 


72.15 


274.9 


905 . 1 


1180.0 


0.1660 


6.024 


305 


310 


77.63 


280 . 1 


901 . 3 


1181.4 


0.1779 


5 . 622 


310 


315 


83.44 


285.2 


897.6 


1182.8 


. 1903 


5 . 254 


315 


320 


89 . 59 


290.4 


893 . 7 


1184.1 


0.2038 


4.907 


320 


325 


96.12 


295.5 


889.8 


1185.3 


0.2177 


4.594 


325 


330 


102.98 


300.6 


885.9 


1186.5 


0.2319 


4.312 


330 


335 


110.25 


305 . 8 


882.0 


1187.8 


. 2476 


4.038 


335 


340 


117.91 


310.9 


878.0 


1188.9 


. 2642 


3.784 


340 


• 345 


126.00 


316.1 


874 . 


1190.1 


0.2813 


3.554 


345 


350 


134.52 


321.3 


870.0 


1191.3 


0.2992' 


3.342 


350 


355 


143.46 


326.4 


865.9 


1192.3 


0.3178 


3.147 


355 


360 


152.89 


331.6 


861.8 


1193.4 


0.3378 


2.960 


360 


365 


162 . 77 


336.8 


857 . 7 


1194.5 


0.3588 


2.787 


365 


370 


173.17 


341.9 


853 . 5 


1195.4 


. 3808 


2.626 


370 


375 


184.08 


347.1 


849.3 


1196.4 


. 4035 


2.478 


375 


380 


195.52 


352.3 


845.1 


1197.4 


. 4275 


2.339 


380 


385 


207 . 49 


357.5 


840.8 


1198.3 


0.4527 


2,209 


385 


390 


220.05 


362.7 


836.6 


1199.3 


0.4789 


2.088 


390 


• 395 


233 . 20 


367.9 


832.2 


1200.1 


. 5060 


1 . 975 


395 


400 


246.9 


373 . 1 


827.9 


1201.0 


. 535 


1.868 


400 


405 


261.3 


378.3 


823.5 


1201.8 


0.566 


1.766 


405 


410 


276.3 


383 . 5 


819 . 1 


1202.6 0.598 


1 . 673 


410 


415 


292.0 


388.7 


814 . 6 


1203.3 ; 0.631 


1.584 


415 


420 


308.5 


394.0 


810.1 


1204.1 ; 0.667 


1.499 


420 


425 


325.6 


399.2 


805 . 6 


1204.8 10.704 


1.421 


425 



54 STEAM-ENGINE INDICATORS 

in the bottom of B, the result would be steam with some water in 
suspension. Steam in this state is known as wet steam, that is, it 
contains some moisture. Expressed in equation form, the number 
of B.T.U. added would be 

Q = q+xr 
in which ;r = the weight of the part vaporized. By means of a steam 
calorimeter, the amount of water held in suspension by the steam 
may be determined. In practice, it amounts to about one to three 
per cent, depending upon the mechanical construction of the plant 
and will average about two per cent. The quality of steam, con- 
sidering saturated steam as unity or one, would then be 100 — 2 = 98 
per cent, dry. So the quantity x is the 98 per cent or the per cent 
of the total amount of water that has been vaporized. 

Superheated Steam. If after saturated or dry steam is obtained, 
additional heat be added by some means until the temperature of 
the dry steam is above that corresponding to the pressure, it is said 
to be superheated. In obtaining superheated steam, more B.T.U. 
have been added than when dry steam was obtained, so another 
expression is used to represent the total heat B.T.U. added, viz, 

Q = q+r+A8 (t-t) 

in which t s equals the temperature of the superheated steam in 
degrees F. and is obtained by the use of thermometers; t equals 
the temperature corresponding to the absolute boiler pressure; and 
.48 equals the specific heat of superheated steam at constant pres- 
sure. This factor varies slightly for different pressures and tem- 
peratures, but for general use the value given is sufficiently accu- 
rate. It may be obtained at any pressure and temperature by 
experiment. 

It is obvious from the foregoing that the number of B.T.U. con- 
tained in one or more pounds of steam, be it wet, dry, or superheated, 
can be obtained by the use of the above formulas and the steam 
tables. 

In order to become familiar with the above formulas and the use 
of the steam tables, a few simple problems will be worked out. It 
must be borne in mind in the use of these tables that whenever a 
pressure is given, the other properties, such as t, q, r, etc., are found 
in Table III; and that if the temperature be given, Table IV must 



STEAM ENGINE INDICATORS 55 

be used. It is also to be remembered that the tables are based on 
absolute pressures, so if the gauge pressure be given instead of the 
absolute pressure, the gauge pressure reading must be converted 
into absolute pressure by adding the atmospheric pressure. For 
example, if 160 pounds gauge pressure, say at sea level, is given instead 
of 160 pounds absolute, then before looking for t, r, etc., correspond- 
ing to that pressure, 14.7 pounds should be added to the 160 pounds, 
making the absolute pressure 174.7 pounds. If the barometric 
pressure is 29.4 inches of mercury when the gauge pressure is 160 
pounds, then the absolute pressure will be 160+ (29.4 X. 49) = 160+ 
14.4, or 174.4 pounds per square inch. This must always be done 
before making use of the steam tables. 

ILLUSTRATIVE PROBLEMS 

Example 1. How many heat units in one pound of water at 160°F.? 

Solution. Looking down the first column of Table IV until 160 degrees 
is found, then following across horizontally, we find in the third column under 
Heat of the Liquid 128.0 B.T.U., which is the number of heat units contained 
in one pound of water at 160°F. 

Example 2. What temperature corresponds to 160 pounds absolute? 

Solution. Since the tables are based on absolute pressures and the 
pressure of 160 pounds is given as absolute, we turn to Table III and follow 
down the first column until 160 pounds is reached, then horizontally across 
to the second column where we find the temperature corresponding to 160 
pounds absolute to be 363.62°F. 

Example 3. What is the heat of vaporization r at 160°F.? 

Solution. Since it is temperature that is given, it is necessary to find 
160 degrees in the first column of Table IV and following across horizontally 
to the fourth column, we find that the heat of vaporization r is 1001.4 B.T.U. 

Example 4. What is the value of r for 160 pounds absolute pressure? 

Solution. Since the pressure is given, it is necessary to look in the first 
column of Table III for 160 pounds, and following across to the fourth column 
we find r to be 858.8 B.T.U. 

Example 5. Steam is made in a boiler at 140 pounds per square inch 
absolute from feed water at a temperature of 70°F., 99 per cent of each pound 
being evaporated. How many heat units are spent in raising the temperature 
of one pound of the water to the boiling point? What are the total number of 
B.T.U. required to make one pound of steam? 

Solution. Looking for q in Table IV, corresponding to the temperature 
of 70°F., we find that it is necessary to interpolate between 90 degrees and 60 
degrees. For 90 degrees, q is 58.1; for 60 degrees it is 28.1. The difference 
is 58.1—28.1, or 30.0 for a difference of 30 degrees. For one degree, the 

value would be — — , or 1.0. Since there is a difference of 10 degrees between 
60 degrees and the temperature of the feed water, we must add to q for 60 



56 STEAM ENGINE INDICATORS 

degrees 1.0X10, or 10.0, making 28.1+10.0, or 38.1. This is the required 
q for 70 degrees, or the number of B.T.U. in one pound of the feed water as 
it enters the boiler. Next, obtain the number of B.T.U. in one pound of the 
water in the boiler after being raised to 140 pounds absolute pressure. In 
Table III the q corresponding to 140 pounds absolute pressure is found to be 
324.4 B.T.U. Since the feed water contained 38.1 B.T.U., the number of 
B.T.U. that has been added in raising one pound of the water from 70 degrees 
to 140 pounds pressure is 324.4-38. 1, or 286.3 B.T.U., the required result. 

Since the steam formed contains some moisture, its quality being 99 
per cent, it follows that Q, the number of B.T.U. required to vaporize one 
pound of the water under the given conditions, would be q+xr. In the first 
part of the problem q was found to equal 286.3 B.T.U. The value of r for 
140 pounds absolute, as obtained from Table III, is 867 . 4. 

Q = 286.3 + (. 99X867. 4) 
= 286.3+858.73 
= 1145.03 B.T.U. 

This result represents the total heat units necessary to maKe one pound of 
steam under the given conditions. 

Feed Water Temperature. The above problem brings out a 
point that has not been noted, viz, the method to follow when the 
temperature of the feed water is other than 32 degrees, q in the 
equation Q = q+xr being the heat of the liquid corresponding to 
the pressure, taking 32 degrees as standard. When the feed water 
at the outset is of a higher temperature than 32 degrees, it is obvious 
that on account of this higher temperature the number of heat units 
required to raise it to the boiling point will be less. It follows, there- 
fore, that the above expression should be modified in order to apply 
to feed water of any temperature. That is, if t is equal to the 'tem- 
perature of the feed water from which the steam is to be made, and 
q 1 equals the corresponding heat of the liquid, the expression for 
Q may be modified so as to read 

Q = q+xr-q 1 
or, taking q+xr= H, the total amount of heat units added is 

Q=H- qi 

Likewise, the formula for superheated steam may be changed 
to the form 

Q = q + r+.48 (*.-*) -ft 
= H+A8(t-t)- qi 

Example. How many B.T.U. must be added to one pound of water at 
177°F. to transform it into steam at 145.5 pounds gauge pressure and a tem- 
perature of 480° F.? 



STEAM ENGINE INDICATORS 



57 



Solution. From inspection, it is evident that the result will be super- 
heated steam, since 480° F. is higher than the temperature corresponding to 
the 145.5 pounds gauge pressure. This being true, the above formula for 
superheated steam must be used. Assuming the atmosphere pressure to be 
14.5 pounds per square inch, the absolute pressure will be 160 pounds. The 
value of q corresponding to this pressure is 335 . 3, r is 858 . 8, t s is 480 degrees, t is 
363.62, and q l is 145.0. Substituting these values in the above formula, we 
have 

Q= 335. 3 +858.8+. 48 (480-363.62) -145.0 
= 1194. 1+55. 86 -145. = 1104. 96 B.T.U 




Fig. 39. Throttling Calorimeter Connected for a Test 



Calorimetric Measurements. In the discussion of the proper- 
ties of steam and the use of the steam tables, the term quality of 
steam was referred to and was used in every instance where saturated 



58 



STEAM ENGINE INDICATORS 



steam was dealt with. Since it is necessary that the quality of 
steam be known in all calculations dealing with the amount of heat 
in a pound of steam, some means must be employed for determining 
the quality. 

Throttling Calorimeter. An apparatus known as a throttling 
calorimeter was devised in the early eighties by Professor C. H. 
Peabody for making this determination. It has been widely used 
since that time and is considered a simple and efficient means for 
obtaining the required results. 

The operation of the throttling calorimeter is based upon the 
principle that saturated steam will become superheated if the pres- 
sure is reduced by throttling without loss of heat. The calorimeter, 
Fig. 39, performs the above function within certain limits, as will 
be evident from a description of its action. It con- 
sists of a closed metallic cylinder K, having a steam 
inlet A and an outlet N; a thermometer well, made 
of suitable material, is provided at T. Two gauges B 
and C are used, B being attached to the calorimeter 
and C to the steam supply line by means of siphons. 
A valve E is placed between the main steam pipe and 
the calorimeter so as to regulate the amount of flow 
of steam into the calorimeter. The nipple A, connect- 
ing the inlet valve E with the chamber K, is made 
of special metal, threaded, and having a well-formed 
orifice, as shown in Fig. 40. 
The connection between the steam pipe and the 
Co g nn 4 ecting F Nip P ie calorimeter should be as short as possible. The cyl- 
inder K and the connections should be thoroughly 
covered with asbestos hair felt, or other nonconducting material, in 
order to reduce the amount of heat radiation. The outlet pipe N 
should be at least 1 inch in diameter for its entire length and it may 
be larger. 

To use the calorimeter, fill the thermometer well T with oil or 
mercury and then insert the thermometer. Attach the gauges and 
permit steam to enter the cylinder K until, say, about 5 pounds pres- 
sure is registered on the gauge B. This pressure should be kept con- 
stant throughout the test by means of the valve E. The siphons 
should be full of water and steam should be permitted to flow through 




STEAM ENGINE INDICATORS 59 

the apparatus for about ten minutes before taking observations. 
The observations to be taken are the pressure P of the steam in the 
main steam pipe, the pressure P x of the steam in the calorimeter, 
the temperature t in the calorimeter, and the barometric pressure P . 
Having this data at hand, the amount of moisture in the steam 
may be determined by combining the two fundamental equations 
Q = q + xr, corresponding to P, the main steam pipe pressure, and 
Q = Qi+ri + C p (t s —t^, corresponding to the pressure P 1 in the cal- 
orimeter. The absolute pressure in the main steam pipe will be 
P+P a , and in the calorimeter Pi+P a . Equating these two expres- 
sions, we get 

q+.xr = q l + r 1 + C p (t s -t) 
Transposing and dividing through by r, we get 



x 



gi + r i + C p (t s -t)-q 



x being 1 minus the per cent of moisture in the steam, or the quality. 
The equation and the method of obtaining the quality of steam 
will be readily understood by the following example. 

Example. The pressure P in the main steam pipe equals 69.8 pounds- 
the pressure P, m the calorimeter equals 12 pounds; the pressure P a of the 
atmosphere equals 14.8 pounds, the temperature t s in the calorimeter equals 
268.2° F. Determine the quality of the steam. 

Solution. The absolute pressure in the steam pipe P+P a = 69. 8 +14. 8, 
or 84.6 pounds. The absolute pressure in the calorimeter P,+P a = 12 + 14. 8,' 
or 26 . 8 pounds. The temperature of saturated steam t x at 26 . 8 pounds = 243 8 
pounds. It is to be noted that'*, is the temperature corresponding to the 
absolute pressure in the calorimeter. The total heat ?,+r, =1161.3 B.T.U ; 
<Z = the heat of the liquid corresponding to P+P a = 286.2, and r for the same 
pressure is 896.9. 

^1161.3+0.48 (268.2-243.8) -286J 
896.9 
= .989 

A throttling calorimeter may be made of pipe fittings, making 
a simple and convenient apparatus which, if properly constructed 
and operated, will give good results. Such an apparatus is illus- 
trated in Fig. 41, which also shows the proper method of connection 
to a steam pipe. Steam is taken from a §-inch. pipe provided with 
a valve and passes through two f -inch tees situated on opposite sides 



60 



STEAM ENGINE INDICATORS 



of a f-inch flange union, substantially as shown in the accompany- 
ing sketch. A thermometer cup, or well, is screwed into each of 
these tees, and a piece of sheet iron perforated with a f-inch hole 
in the center is inserted between the flanges and made tight with 
rubber or asbestos gaskets, which also act as nonconductors of heat. 
For convenience a union is placed near the valve as shown, and the 
exhaust steam may be led away by a short lj-inch pipe, shown by 
dotted lines. The thermometer wells are filled with mercury or 
heavy cylinder oil, and the whole instrument from the steam main 
to the 1 f-inch pipe is well covered with hair felt. 




Fig. 41. Throttling Calorimeter Made of Pipe Fittings 



Great care must be taken that the f-inch orifice does not become 
choked with dirt, and that no leaks occur, especially at the sheet 
iron disk, also that the exhaust pipe does not produce any back pres- 
sure below the flange. Place a thermometer in each cup, and open- 
ing the |-inch valve wide, let steam flow through the instrument for 
ten or fifteen minutes; then take frequent readings on the two ther- 
mometers and the boiler gauge, say at intervals of one minute. 

Separating Calorimeter. Another type of calorimeter some- 
times used in cases where the steam contains from 5 to 10 per cent 
of moisture, is the separating calorimeter. It works upon the prin- 
ciple that the moisture contained in the steam is liberated by mechan- 
ical means. In its usual form, the calorimeter consists of a cylin- 
drical vessel so constructed that the moisture is separated from the 



STEAM ENGINE INDICATORS 61 

steam and returned, the dry steam passing to a condenser where it 
is collected and afterward weighed. The separating vessel is pro- 
vided with a glass gauge and graduated scale which indicates the 
weight of the moisture taken out of the steam. Having obtained 
the weight W of the separated water and the weight W 1 of the dry 
steam, then the percentage of moisture to the total amount of the 
liquid would be 

W 

y ~w+w 1 

Therefore, the percentage of dry steam would be 

x=l—y 

W 

= 1 



w+w, 

Example. Required the weight of steam in the cylinder of a 16 X 36- 
inch engine when the piston has moved on its stroke 27.4 per cent of the dis- 
tance from the h.e. The steam in the cylinder at this instant is under a pres- 
sure of 114 pounds gauge, as determined from the indicator card. The piston 
displacement for the h.e. is 4.188 cubic feet and the clearance on the h.e. is 
5.2 per cent. The atmosphere pressure is 14.5 pounds per square inch. 

Solution. The first thing required is the volume of steam back of the 

piston, when the engine has made 27.4 per cent of the stroke. To 27.4 per 

32 6 
cent add the clearance 5.2 per cent, making a total of 32.6 per cent, or —rr of 

1U0 

the whole volume of the cylinder containing steam at the instant under con- 
sideration. Since the total piston displacement for the h.e. is 4.188 cubic feet, 

32 6 
then the volume of steam to be considered would be 4.188 X — — , or 1.36 cubic 

100 

feet. The absolute pressure of the steam in the cylinder would be 114 + 14.5 

pounds, or 128.5 pounds per square inch. Looking in Table III for the weight 

of 1 cubic foot of steam at 128.5 pounds absolute pressure, we find it to be by 

interpolation .2867 pounds. Since there are 1.36 cubic feet in the cylinder 

at 27.4 per cent of the stroke, the weight of steam in the cylinder at the instant 

in question would be 1 . 36 X . 2867, or . 3899 pounds. 

Volume and Weight of Steam. In considering any problem 
dealing with the weight of steam in the cylinder or the piston dis- 
placement, the per cent of clearance must always be taken into 
account, as in the problem above. 

In studying and analyzing the performance of an engine, it is 
often desirable to obtain the volume and the weight of steam in the 
cylinder from the indicator card at the several events, and also to 



62 STEAM ENGINE INDICATORS 

know the quality of the steam at these points. From the study of 
the indicator cards and the steam tables, we are now prepared to 
obtain these several values. 

Volume of steam at c.o. in cubic feet is equal to the piston displace- 
ment in cubic feet multiplied by the per cent of c.o. plus the per cent 
of clearance. For example, in the problem given above, the h.e. 
displacement was 4.188 cubic feet and the h.e. clearance was 5.2 per 
cent. It is desired to obtain the volume of steam at c.o. which takes 
place at 34.8 per cent. The sum of the clearance and c.o. per cents 
is 40. Therefore, the volume of steam at c.o. is 4.188X40 per cent, 
or 1.675 cubic feet. 

Volume of steam at release in cubic feet is the product of the pis- 
ton displacement and the sum of the per cents of release and clear- 
ance. 

Volume of steam at compression is found by multiplying the pis- 
ton displacement by the per cent of compression plus the per cent 
of clearance. 

Weight of steam at c.o. is the product of the weight of 1 cubic 
foot of steam at the absolute pressure at c.o. and the volume of steam 
in cubic feet at c.o., both as obtained from the indicator card. 

Weight of steam at release is the product of the weight of 1 cubic 
foot of steam at the absolute pressure at release and the volume of 
steam in cubic feet at release. 

Weight of steam at compression is found in the same manner as 
that at release. 

Re-evaporation or condensation per revolution in pounds is the 
weight of steam at release minus the weight of steam at c.o. If the 
answer is positive, it indicates that there is a re-evaporation, and if 
negative, a condensation. 

Re-evaporation or condensation per i.h.p. per hour in pounds is 
the item just given multiplied by the revolutions per hour and divided 
by the i.h.p. 

Weight of steam per revolution, as determined by weighing, is the 
total weight of steam used by the engine divided by the total num- 
ber of revolutions. 

Weight of mixture in the cylinder per revolution in pounds is the 
weight of steam per revolution as determined by weighing plus the 
weight of steam at compression. 



STEAM ENGINE INDICATORS 63 

Per cent of mixture accounted for as steam at c.o. is one hundred 
times the weight of steam at c.o. per revolution, divided by the weight 
of the mixture in the cylinder per revolution. 

Per cent of mixture accounted for as steam at release is one hundred 
times the weight of steam at release per revolution divided by the 
weight of the mixture in the cylinder per revolution. 

Thermal Efficiency. Having obtained a working knowledge of 
the properties of steam from the preceding discussion and the prob- 
lems dealing with the B.T.U. values of steam, we are now ready to 
consider the thermal efficiency of an engine, but before this can be 
calculated, several things must be known. 

(1) The amount of work done in a unit of time. 

(2) The weight of steam used by the engine in the same length of time. 

(3) The number of B.T.U. in each pound of steam used. 

These quantities must be accurately determined while the engine 
is in operation. 

Example. To illustrate what is meant by thermal efficiency, assume 
an engine which m developing 242 h.p. uses 13,000 pounds of steam in two 
hours; steam pressure 186.3 pounds gauge; quality of steam 99 per cent; tem- 
perature of feed water 60°F.; and atmosphere pressure 14.7 pounds. Find 
the thermal efficiency in per cent. 

Solution. The number of foot pounds of work done in a minute is 
242X33,000 = 7,986,000. The number of B.T.U. in one pound of steam at 
186.3 pounds gauge, which is 200 pounds absolute, is q+xr- qi =S54: 3 + 
(843. 5 X. 99) -28.1, or 1161.27 B.T.U. Since the engine is using 13,000 
pounds of steam in 2 hours, the amount of steam being used in one minute will 
^2^60' ° r 108,333 - The corresponding number of B.T.U. supplied per min- 
ute will be 108.333X1161 .27, or 125,804.25. Changing this to the equivalent 
toot pounds of energy by multiplying by 778, we get 125,804.25X778, or 
97,875,706 . 5 as the total energy in foot pounds supplied the engine per minute 
Therefore, the thermal efficiency is 

E _ energy delivered per minute X 100 
energy supplied per minute 
_ 7,986,000X10 
" 97,923,444.58 =815 per cent 

The thermal efficiency is expressed by some as the B.T.U. sup- 
plied per minute per i.h.p. instead of per cent. Applying this to 
the problem above, we get for the thermal efficiency 
E _ 13000X1161.27 
2X60X242 
= 519.5 B.T.U. 



64 STEAM EXGIXE INDICATORS 

Generally speaking, the efficiency of a steam engine is spoken of 
as being so many pounds of steam per i.h.p. per hour. In the prob- 
lem under consideration, this would give an efficiency 

„ 13000 



2X242 
= 26.8 pounds of steam per i.h.p. per hour 

The B.T.U. per i.h.p. per minute varies inversely as the thermal 

efficiency, so if one value is known, the other can be easily obtained 

by using the two constants — 778 the mechanical equivalent of heat, 

and 33,000 the number of foot pounds per minute which constitutes 

a horsepower. If an engine has a thermal efficiency of 100 per cent, 

33000 
it would require s or 42.42 B.T.U. per h.p. per minute. An 

778 

engine which used 520.1 B.T.U. per minute, as in the above example, 

42.42 
has a thermal efficiencv of rrTr-XlOO, or 8.17 per cent. 

INTERPRETATION OF INDICATOR CARDS 

Theoretical Diagram. As a basis of comparison between indi- 
cator diagrams taken from the same engine under different condi- 
tions, from different engines, and for design purposes, a theoretical 
diagram is constructed on the assumption that the expansion curve 
of a theoretically perfect engine would be that of a hyperbola. 
Experiments conducted at various times and on a large number of 
engines substantiate the assumption. The hyperbolic curve has 
the property that the product of the distances of any point on the 
curve from the line of zero volume is constant. This when expressed 
in equation form is 

p 1 V l =C (constant) 

in which P 1 is pressure at c.o. and V x is volume at c.o. If P 2 is 
pressure at release and V 2 is volume at release, then 

P 2 V 2 = C (constant) 

It follows then that P l V l = P 2 V 2 . In this equation, P l and P 2 are 
absolute pressures and V x and U 2 include the clearance volume. 



STEAM ENGINE INDICATORS 



65 



To Draw the Theoretical Card. To draw an ideal diagram (see 
Fig. 42), draw P X equal to the length of stroke and P equal to the 
clearance. Draw Fand PA perpendicular to OX and draw Y S 
parallel to X and at a height corresponding to the boiler pressure. 

The line of initial pressure A C is then drawn parallel to Y S 

and is usually taken as from 90 to 95 per cent of the boiler pressure, 

if there is no special cause for loss. Then take A C as the portion 

OX 

of the stroke at which steam is admitted, so that equals the ratio 

OR 4 

of expansion. The expansion line is considered a hyperbolic curve 

with Y and X as asymptotes. To draw the hyperbolic curve ; 




Fig. 42. Ideal Indicator Card 

first draw the line AC B parallel to the atmosphere line and F D B 
and RC perpendicular to it. Then make points 1, 2,3, 4> etc., on 
C B and connect them with the point 0. At the points V ', 2' , 3' , 4', 
etc., where these lines intersect the line R C, draw parallels to C B 
until they meet perpendiculars from points 1,2,3, 4, etc. The points 
of intersection of these lines are points on the hyperbolic curve C D, 
as shown in Fig. 42. Any number of points may be used, but there 
must be enough to determine the curve. A theoretical compression 
curve may be drawn in the same manner as an expansion curve, 
letting the perpendicular to the atmosphere line be drawn from the 
point of compression instead of from the point of cut-off. 

The area AC DM N II is the theoretical card, with a given 



66 STEAM ENGINE INDICATORS 

boiler pressure and an assumed drop and ratio of expansion. The 
actual card for the same data would probably appear more nearly 
like the shaded area which lies mostly within the outline of the theo- 
retical card. In designing engines, it is well to know the ratio of 
the actual to the ideal card for all types of engines. This ratio 
varies between .5 and .9 according to the speed, type of engine, and 
kind of valves. 

It will be observed that the actual expansion curve does not 
coincide with the theoretical curve in Fig. 42. It is a well-known 
fact that, in the cylinder of a steam engine, the temperature of the 
steam changes during the stroke. Usually, the piston and valves 
leak steam more or less ; initial condensation takes place at the begin- 
ning of the stroke and re-evaporation at the end of the stroke. These 
factors cause a variation from the true theoretical curve. The 
object, therefore, in constructing the theoretical diagram is to ascer- 
tain where and to what extent these variations occur and to study 
the causes of the irregularities to the end that the necessary adjust- 
ments may be made to eliminate the errors in so far as possible. 

In the construction of the theoretical indicator diagram, it is 
assumed that no loss of heat occurs in the cylinder. It is a well- 
known fact that as the steam enters the cylinder, some is condensed 
on account of the comparatively cool cylinder walls. Toward the 
end of the stroke, the cylinder walls give off heat with the result 
that either all or a part of the condensed steam is re-evaporated. 
Hence, the expansion curve of the theoretical diagram would natur- 
ally fall below that of the actual curve near the end of the stroke. 
Speaking in general, a close approximation of the two curves is an 
indication of good valve adjustment and economical steam distri- 
bution. It is, therefore, advantageous to draw the theoretical dia- 
gram in order to have something upon which to base an opinion as 
to the condition of the engine. It would be well when not satisfied 
with the performance of an engine to construct theoretical indicator 
cards and compare them with actual cards. 

Steam Cards Showing Miscellaneous Troubles. From our 
study of the indicator diagram, it is evident that a great deal of use- 
ful information may be obtained by the correct interpretation of 
them. Fundamentally, the diagram is to register pressures for 
given piston positions, so all the information that is obtained in 



STEAM ENGINE INDICATORS 67 

addition to this, comes from a source of reasoning. A few cards, 
illustrating information which may be obtained, are given in Figs. 
43 to 64, inclusive. 




Fig. 43. Diagram Showing Improper Valve Lubrication 

Valve Trouble. Figs. 43 and 44 illustrate cards taken from the 
h.e. of one of the cylinders of a locomotive running at thirty miles 
per hour, using 240 pounds steam pressure, with the reverse lever 
placed in the second notch ahead of the center position. This loco- 




Fig. 44. Improvement in Diagram by Uae of Lubricant 

motive has a superheater, hence, with the high boiler pressure and 
superheat, trouble was experienced with the lubrication of the valves. 
This is indicated by the reduced area and distorted card shown in 
Fig. 43 as compared with that illustrated in Fig. 44. The card 




Fig. 45. Diagram Showing Sticky Indicator Piston 

shown in Fig. 44 was obtained about twenty minutes later than that 
illustrated in Fig. 43. In Fig. 44 lubricating oil had been forced into 
the steam chest. The effect on the card shown in Fig. 43 was caused 
by the valve clinging to its seat, resulting in a shorter travel and poor 
steam distribution. 



68 STEAM ENGINE INDICATORS 

Sticky Indicator Piston. Figs. 45 and 46 show cards obtained 
from the c.e. of one of the cylinders of the same locomotive. The 
wavy appearance of the steam line in Fig. 45 is due to a dry, sticky 




Fig. 46. Diagram After Trouble of Fig. 45 has been Removed 

indicator piston. Fig. 46 illustrates the appearance of the steam line 
after the indicator piston had been removed, well oiled, and replaced. 




Fig. 47. Distorted Card Due to Binding Indicator Piston 

Tight Indicator Piston. The cards exhibited in Figs. 47, 48, and 
49 illustrate the distortion of the card which may occur when the 
indicator piston does not fit properly and binds, due to the indicator 




Fig. 48. Distorted Card Due to Binding Indicator Piston 

parts not being put together properly. The indicator by means of 
which these diagrams were obtained had the screw in the bottom of 
the piston run up so far that the piston rod did not fit down over the 
projection on the piston, hence perfect alignment was not obtained. 



STEAM ENGINE INDICATORS 



69 



Of the three cases, Fig. 40 is the worst. The area of the card is very 
much decreased and the back-pressure line is high. 

Lost Motion. The effect of lost motion in the connections of an 
indicator is apparent in the cards illustrated in Figs. 50 and 51. It 



Fig. 49. Bad Case of Binding Indicator Piston 



is, perhaps, most noticeable in the wave in the expansion line and 
the height of the back-pressure line. 




Fig. 50. 



Effect on Diagram of Lost Motion 
in Indicator Connections 



Variable Cut-Off. In Fig. 52 is shown a card taken from a Buck- 
eye engine at a speed below that at which the governor sets. With 
the engine working under this condition, the greatest c.o. is obtained. 




Fig. 51. 



Diagram Showing Effect of Lost Motion 
in Indicator Connections 



Fig. 53 illustrates a card taken from the same engine, running at 200 
r.p.m. and with a load slightly under full load. Fig. 54 illustrates 
another card taken from the same engine operating under a very light 
load. The c.o. occurs very early. The small area of the card sug- 
gests the small amount of work being done in the cylinder. 



70 STEAM ENGINE INDICATORS 

Long Indicator Cord. Fig. 55 illustrates the effect of too long an 
indicator cord. Comparing this diagram with those shown in Figs. 
52, 53, and 54 taken from the same engine but from the other end 




Fig. 52. Card from Buckeye Engine at Low Speed 

of the cylinder, the distortion becomes very apparent. Fig. 56 
illustrates a distorted card from the same engine, its distortion being 
due to the cord slipping off from the sector of the reducing motion. 




Fig. 53. Card from Buckeye Engine at Nearly Full Load 

It is to be noted that there is a small loop in ^the top of this card 
which indicates too much compression. Oftentimes this loop 
appears when there is nothing wrong with the indicator or its attach- 
ments, but is an indication of disarrangement of the valves of the 
engine. 




Fig. 54. Card from Buckeye Engine for Very Light Load 

Speed Governing. There are two ways of governing the speed of 
an engine, namely, by throttling the steam or by varying the point 
of c.o. to suit the load conditions. The effect of these two methods 
on the indicator diagram is shown in Figs. 57 and 58. The diagram, 
Fig. 57, was taken when the speed was maintained constant by 
changing the point of c.o., this being decreased as the load decreased, 



STEAM ENGINE INDICATORS 71 

thus reducing the power in the cylinder. The card in Fig. 58 was 
obtained when the speed of the engine was maintained constant by 
throttling the steam supply rather than by changing the point of c.o. 




Fig. 55. Card Showing Effect of Long Indicator Cord 

It should be noted that the area of the card is reduced in the same 
manner as when the point of c.o. was changed but that the events of 
the stroke remain unchanged under all conditions of throttling. This 




Fig. 56. Card Showing Slipping Indicator Cord 

is not true, however, of the cut-off governor, because in this type, by 
changing the point of c.o., the other events of the stroke are affected 
in some degree. 




Fig. 57. Card Showing Effect of Changing Cut-Off 

Faulty Valve Arrangement. A typical card taken from a Straight 
Line engine, running at 270 r.p.m. at full load, is shown in Fig. 59. 



72 



STEAM ENGINE INDICATORS 



Aside from illustrating the form of card that this particular type of 
engine gives, it is of interest because it indicates a faulty valve arrange- 
ment. Referring to the figure, it will be seen that admission occurs 




Fig. 58. Effect on Card by Throttling the Engine 

at the end of the stroke as indicated at a. Late admission is indi- 
cated by the sloping admission line, giving the space b between the 




Fig. 59. Effect of Faulty Valve Arrangement 



end of the stroke and the point where full admission occurs. Early 
admission would be indicated in the same way, the exception being 




Fig. 60. Card of Gas Engine Operating Under Full Load 

that the admission line would slope towards the line x y drawn at the 
end of the card, instead of away from it, as it does in the case illus- 
trated. When retarded admission occurs in a very large degree, the 
curvature of the admission line is more pronounced. 



STEAM ENGINE INDICATORS 73 

Gas Engine Cards. It seems desirable to show a few typical 
gas engine cards, as every engineer may be called upon to indicate 
gas engines as well as steam engines. Fig. 60 is a diagram obtained 
from a gas engine operating under a full load of approximately 18 h.p. 
A 240-pound spring was used in taking the card. Fig. 61 is a dia- 
gram taken from the same engine, but operating under different con- 
ditions. It shows the change in the diagram produced by throttling 
the mixture for various loads. This card also shows that the indi- 
cator cord stretched slightly, otherwise the different maximum com- 
pression points a would have fallen on a line perpendicular to the 
atmosphere line. 

In most of the diagrams presented thus far, the errors pointed 
out were those due chiefly to defects in the operation and attach- 
ment of the indicator, or in lubrication. 




Fig. 61. Change in Gas. Engine Diagram by 
Throttling the Mixture for Various Loads 

Cards Showing Valve Troubles. It is now desired to direct 
attention directly to defects in valve-setting as shown by the 
indicator diagram, to the end that suggestions may be given as to how 
to properly adjust the valves of an engine by the use of an indicator. 

The most common faults in the distribution of steam in an 
engine cylinder can be divided into four classes, viz, admission too 
early or too late; cut-off too early or too late; release too early or 
too late; and compression too early or too late. 

Late Admission. The diagram, Fig. 59, shows too late admis- 
sion, as was previously pointed out. If a plain slide valve were used, 
the reason why admission occurred too late was because the angle 
of advance was too small. If admission seems too early, the oppo- 
site thing is true and the angle of advance should be decreased. 

Excessive Back Pressure. The cards shown in Figs. 47, 48, and 
49 portray too much back pressure. While in these cards it was due 
to defects in the indicator, rather than in the engine, yet this exces- 
sive back pressure is sometimes found due to inherent defects in the 



74 STEAM ENGINE INDICATORS 

design of the engine, such as too small exhaust ports or pipes, or to 
the passage of steam through coils of pipe for heating purposes. 
Excessive back pressure is an indication of a loss of power and should 
be kept as small as possible. If the exhaust steam is used for some 




Fig. 62. Diagram Showing Effect of Too Early Cut-Off 

useful work, such as heating, etc., an increased back pressure above 
the normal is permissible. 

Early Cut-Off. The diagram, Fig. 62, shows the c.o. to come too 
early. In this case the c.o. is so early that the expansion line extends 
below the atmosphere line, making a loop. In finding the area of 
such a card for computing the power, the area of the loop must be 
subtracted from the total area. In using a planimeter to determine 
the area, it will automatically make the reduction so the reading will 
be correct. This loop is frequently spoken of as negative work. 

Wire Draiving. Fig. 63 shows a pair of diagrams from a plain 
slide-valve engine. The admission lines are good. The sloping 
steam lines show wire drawing due to the slow action of the valve 




Fig. 63. Pair of Diagrams from Plain 
Slide-Valve Engine 

or too small ports or pipes. This wire drawing decreases the area 
of the card and indicates a loss. The greatest fault is the inequal- 
ity of area of the diagram. The late cut-off and consequent late 
compression of one end causes more area than the too early cut-off 



STEAM ENGINE INDICATORS 75 

and too early compression of the other end. These cards can be 
improved upon by adjusting the angle of advance of the eccentric 
and the length of the valve rod. If the left card were a normal one 
the hook at A might indicate an open cylinder cock. 

Early Compression. The diagram of Fig. 64 indicates too early 
compression. The compression curve extends above the initial 
pressure line. The area of the loop must be subtracted from the card 
area when computing the i.h.p. If the cut-off is kept the same and 
the compression made what it should be, the gain in area would be 
the area included between the full line and the dotted line plus 
the area of the loop. The remedy for this case is to decrease the 
inside lap, which would permit exhaust to occur earlier and com- 
pression later. 

The amount of compression an engine should have varies with 
the speed and type. Slow speed engines require less compression or 




Fig. 64. Diagram Showing Early Compression 

cushioning than high speed engines. The exhaust steam should 
never be compressed higher than the boiler pressure. 

If the valve travel is increased, compression is retarded — that 
is, decreased — and release occurs sooner. 

TESTING STEAM ENGINES 

In the beginning of this study, it was stated that the indicator 
had been largely responsible for the refinement of the modern steam 
engine. In what way the indicator has influenced the development 
will be evident from the suggestions which follow and from the work 
involved in the testing of engines. The testing of steam engines 
requires considerable preliminary work and very careful attention 
to details. The tests may be made to ascertain whether the valves 



76 STEAM ENGINE INDICATORS 

are properly set; to determine i.h.p., b.h.p., and f.h.p.; to determine 
the amount of steam used per i.h.p. per hour, or the commercial 
efficiency; and to investigate the transference of heat between the 
steam and the cylinder walls, and losses due to this transference. It 
should be borne in mind that most of the results sought for are 
closely allied, so that one complete test may give data sufficient to 
obtain the value of all the factors mentioned. For instance, if one 
is seeking the loss due to friction, he must obtain the b.h.p. and i.h.p. 
and, having these, it is an easy matter to obtain the mechanical 
efficiency. 

Factors Considered. Usually the principal object in testing a 
steam engine is to determine the cost of power or the effect of such 
conditions as superheating, jacketing, varying the point of cut-off, 
varying the point of compression, clearance, steam pressure, etc., 
upon the steam economy of the engine. We must determine, there- 
fore, first, the cost of fuel, and second, the actual amount of heat used. 
In either case, the horsepower of the engine must be determined. 

The indicated power is determined by means of the indicator, 
and the actual power delivered, by means of a dynamometer or fric- 
tion brake. To determine the cost of power in terms of coal, it is 
necessary to conduct a careful boiler test, usually of twenty-four 
hours duration. 

When the cost is expressed in terms of steam per horsepower 
per hour, we may follow either of two methods, viz, we may con- 
dense and weigh the exhaust steam, or we may weigh the feed water 
supplied to the boiler. When the object of the test is primarily for 
an investigation of the performance of the engine, it is best to weigh 
the condensed steam. This is the method used in the test described 
herein. An hour under favorable conditions is usually sufficient for 
such tests. Steam used for any purpose other than running the 
engine must be determined separately and allowed for. 

Probably the most accurate terms in which to state the per- 
formance of an engine is in B.T.U. per horsepower per minute. When 
the cost is expressed thus, it is necessary to measure the steam pres- 
sure, amount of moisture in the steam, and temperature of condensed 
steam when it leaves the condenser. Jacket steam must be accounted 
for separately. Engines with their boilers, etc., for large plants, are 
usually built under contract to give a certain efficiency, and their 



STEAM ENGINE INDICATORS 77 

fulfillment of this contract can be determined only by a complete 
test of the entire plant. Before beginning the test, the engine should 
be run for a sufficient length of time in order to limber it up and get 
it thoroughly warmed. It is of the utmost importance that all con- 
ditions of the test should remain constant, especially the boiler pres- 
sure and the load. All instruments used in the test should be cali- 
brated before being used, in order to determine the effect of any 
errors to which they may be subject. 

Thermometers. All important temperatures, such as feed 
water, injection water, condensed steam, etc., must be taken by accu- 
rate thermometers, the errors of which have been previously deter- 
mined and allowed for. Good thermometers sold by reliable dealers 
are usually satisfactory. Cheap thermometers are of little value in 
an engine test. 

Indicators. The most important and in many respects the least 
satisfactory instrument used in the test is the indicator. It is sub- 
ject to an error of 2 to 3 per cent, depending on the conditions. It 
does not work satisfactorily at more than 400 revolutions per minute. 
If the indicator is carefully tested under conditions similar to those 
under which it is used, the errors may be reduced to a minimum, but 
there will always be some uncertainty. The principal errors to which 
the indicator is subject have already been mentioned. 

Scales. Weighing should be done on standard platform scales. 
The water may be weighed in barrels provided with large quick- 
acting drain valves which will allow the water to run out quickly. 
It is seldom possible to drain barrels completely, and so it is best 
to let out what will run freely, then shut the valve and weigh the 
barrel. This we call "empty" weight, and deducted from the weight 
"full" evidently gives us the true weight of water. 

If not convenient to weigh the water, it may be measured in 
tanks or receptacles of known capacity, and the temperature taken, 
allowing the proper weight per cubic foot for water at that tempera- 
ture; or it may be determined by meters. 

Meters. Water meters are of two kinds, viz, those that record 
the amount of water by displacement of a piston, and those in which 
the flow is recorded by means of a rotating disk. Piston water 
meters can be made very accurate, and if working under fair condi- 
tions of service, they may be relied upon to a close degree. The 



78 STEAM ENGINE INDICATORS 

chief error in a meter arises from the air that may be in the water. 
To reduce this error to a minimum, the meter should be vented so 
as to allow the air to escape without passing through the meter. 
Rotary meters are good enough for very rough work, but are seldom 
sufficiently accurate for a careful engine test. So far as possible 
weirs should not be used in engine work. They may be fairly accu- 
rate under certain conditions, but a very little oil in the water may 
affect them seriously. They may sometimes be used to measure the 
discharge from a jet condenser, for then the volume is so large that 
the actual error is proportionately small. The use of meters for 
testing purposes should always be discouraged. When used, how- 
ever, they should always be carefully calibrated under as nearly as 
possible the same conditions as existed during the test. 

Gauges. Pressures should be measured on good gauges that 
have been recently tested. The atmospheric pressure should be 
read from the barometer, and for accurate work this pressure should 
be used. For ordinary work, 30 inches of mercury, or 14.7 pounds, 
may be used. 

Calorimeters. When using superheated steam, it is sufficient 
to take the temperature and pressure in the steam pipe, but if sat- 
urated steam is used, we must determine the amount of moisture it 
contains. This is done by means of a calorimeter such as has pre- 
viously been described. 

Prony Brakes. Any of the forms of friction brake described 
will answer the purpose. For smooth and continuous running, it 
is essential that the brake and its band be cooled by means of water 
and that some lubrication be applied to the surface of the brake wheel. 
The water may either circulate in the rim of the wheel or around the 
brake band, but it must not come in contact with the rubbing surfaces. 

Original Type. The most common form of brake used is some 
modification of the Prony brake as illustrated in Fig. 65. This is 
one of the simplest forms of absorption dynamometer. The two 
wood blocks A and C are held together against the rim of the 
pulley P by bolts. The thumb nuts, E E, are used to adjust the 
pressure. By means of the bolts, the arm L is held to the upper 
block. From this arm is suspended the ball weight B which, by 
sliding along the arm, counterbalances the weight of the arm and 
pan at the other end. The pulley revolves at the required speed in 



STEAM ENGINE INDICATORS 



79 



the direction indicated by the arrow. The bolts are tightened until 
the lever remains stationary in a horizontal position when a known 
weight W is hung at the end. Suitable stops must be arranged at 




JZ3_ 



■tar 



Fig. 65. Original Form of Prony Brake 



the outer end of lever L to prevent an accident in case the brake 
should happen to grip the wheel and cause the weight W to be thrown 



over. 



The amount of work absorbed by the brake depends upon 
the weight W, the length R, and the speed. It is independent of 
the diameter of the pulley and the pressure of the blocks because the 
moments of forces about the center of the pulley are equal when the 




Fig. 66. Rope Form of Prony Brake 

lever L is horizontal. Letting / equal the coefficient of friction, p 
the pressure of the blocks, and r the radius of the pulley, we have 

fpr = WR 



80 STEAM ENGINE INDICATORS 

The work done at the face of the pulley equals the tangential 
force between the block and the wheel multiplied by the distance 
passed over, which also equals weight W multiplied by the number 
of feet W would move through if it were free to rotate. 

Let N be the number of revolutions per minute. Then the dis- 
tance passed through per minute equals 2n rN and the work done 
equals 2tz rNfp. Then as fpr = W R, the work done at the rim 
of the pulley equals the left-hand side of the equation multiplied 
by 2 7i N, and to keep both sides equal we multiply WR by 2 n N. 
Then the work done per minute is obtained from the expression 
2nNWR. 

2ttNWR 
b - h ' p - = "^^~ 

= .0001904 NWR 

Example. A Prony brake having an arm 4 feet long attached to the 
pulley of an engine sustains a weight in the scale pan of 50 pounds ^hen the 
speed of the engine is 300 r.p.m. Find the brake horsepower. 

b.h.p. = .0001904 X300 X50 X4 
= 11.424 

Rope Type. The rope brake shown in Fig. 66 is easily con- 
structed of material at hand and being self-adjusting needs no accu- 
rate fitting. For large powers, the number of ropes may be increased. 
It is considered a most convenient and reliable brake. In Fig. 66 
the spring balance B is shown in a horizontal position. This is not 
at all necessary; if convenient the vertical position may be used. 
The ropes are held to the pulley or flywheel face by blocks of wood 
0. The weights at W may be replaced by a spring balance if desir- 
able. 

To calculate the brake horsepower, subtract the pull registered 
by the spring balance B from the load at IF. The lever arm R is 
the radius of the pulley plus one-half the diameter of the rope. The 
formula for power absorbed is 

2r.RN{W-B) 

b.h.p. = 

33000 

= .0001904 R N (W-B) 



STEAM ENGINE INDICATORS 



81 



If B is greater than W, the engine is running in the opposite 
direction. In this case the formula becomes 

b.h.p. = . 0001904 R N (5- W) 

Example. A rope brake is attached to a gas engine brake wheel. The 
average reading of the spring balance is 8 pounds when W is 80 pounds. If 
the radius of the brake wheel is 28 inches and the rope 1 inch in diameter, what 
is the b.h.p. when the engine makes 350 revolutions per minute? 

28 5 
R = 28 + § = 28 \ inches = — ^ feet 

Then from the equation for brake horsepower, we have 
b.h.p = .0001904 R N (W-B) 

28 5 
= .0001904 X— a <2 X350 

i — 

= 11.4 

If both the indicated horsepower and the brake horsepower are 
known, the power lost in friction may be found by subtracting the 
b.h.p. from the i.h.p. 

Modern Band Type. The two forms of brakes shown in Figs. 
65 and 66 serve their purpose very well but are not very durable. 




Fig. 67. Modern Band Form of Prony Brake 

When it is desired to make repeated tests of an engine for a consider- 
able period of time, or when it is desired to keep the machine in readi- 
ness for tests at all times, as in experimental engineering labora- 
tories, it is better to provide a brake of the form illustrated in Fig, 



82 



STEAM ENGINE INDICATORS 



67. This brake is made up of two metal straps CC lt as shown in the 
cross-sectional "view. Attached to these metal straps are a number 
of wood blocks placed at regular intervals. These blocks are 
made of hardwood and form the rubbing medium of the brake. 
The brake band and blocks are held in place on the pulley by hav- 
ing metal clips extending down the side of the pulley for a fractional 
part of an inch. The brake is tightened up by means of the hand 
wheel E. The pipe F x delivers water to the rim of the wheel 
for keeping it cool. Pipe F 2 is arranged to scoop up the water 
from the rim, thus keeping the rim of the wheel filled with cool water. 
Ordinarily pipe F 2 is not needed. The water in the rim will never 
be heated above the boiling point and this temperature will do no 
harm. When the engine is running in the direction indicated by 




Fig. 68. Revolution Counter and Recorder 



the arrow, the tendency is for the brake band to move in the same 
direction, but the V-shaped arms resting upon the platform scales 
prevent this, and the amount of pressure W exerted by the brake 
lever is weighed by the scales. Hence, one can at any time easily 
determine the work delivered to the brake. This form of brake is 
shown in application on a Buckeye engine in Fig. 10; the scales are 
not used, but instead the brake arm is connected to a chain, which 
runs over a quadrant to which it is attached. Attached to this 
quadrant is an arm that carries a weight B and a pointer E. The 
pointer indicates the pounds pull on the graduated arc C. By care- 
ful calibration, the arc C is graduated in pounds. In the brake shown 
in Fig. 67, the pressure W on the scale must be corrected before 
using the brake horsepower formula, for the unbalanced weight of 
the brake arm. If the brake band is supported on a knife edge imme- 



STEAM ENGINE INDICATORS 



83 



diately above the center of the engine shaft, and the outer end of 
the shaft then weighs W x pounds, the brake horsepower formula 
would be 

b.h.p. = . 0001904 RNiW-WJ 

Speed Counter. In finding the b.h.p. or i.h.p. of an engine, 
it is necessary to know the number of revolutions the engine makes in 
a minute. This speed is usually designated as r.p.m. In order to 
obtain the correct r.p.m., an instrument known as a revolution 
counter is usually attached to some revolving or reciprocating part 
*"f the engine. A common form of such a counter is shown in Fig. 68. 




Fig. 69. Standard Form of Speed Counter 



The actuating motion of the engine or other machine to which 
the counter is to be attached, is generally communicated by a rod 
or bar moving in the same general direction of its length, and the 
lever should be connected to such rod at a right angle when such 
rod is in the middle of its movement. It should not be clamped 
rigidly to the shaft until the latter is turned so as to bring the pawl 
to the middle of the stroke. It may be determined, practically, by 
opening the lid of the counter and watching the movement of the 
pawl as the shaft is rotated, when the middle point of its travel can 
be easily fixed. When in this position, clamp the crank to the shaft 
by means of the set screw. 

This arrangement provides for the utilization of the entire 
motion of the actuating rod at the angle of greatest effectiveness in 
moving the mechanism of the counter; and if for any reason the 



84 STEAM ENGINE INDICATORS 

movement of the rod is shorter than its longest possible stroke — 
as might happen in the case of a direct acting pump — there would 
still be ample motion to insure a correct count. 

This counter is adapted to either right or left rotary or recipro- 
cating motions and is capable of 500 revolutions per minute with 
safety to the machine and accuracy in the enumeration. 

The shaft through which the actuating force is applied may 
extend from the counter either on the right-hand or left-hand side, 
as desired. 

A very simple form of speed counter is illustrated in Fig. 69. 
It has a rubber tip which is held in the center of the engine shaft. 
The motion of the engine shaft is transmitted to the shaft of the 
counter which drives through a system of gears a pointer, the latter 
indicating on a graduated dial the number of revolutions made in a 
given time. When well made, this is a very accurate instrument 
and may be read with reliability for speeds up to as high as two or 
three thousand r.p.m. 

INDICATOR TROUBLES AND REMEDIES 

Necessity for Care in Using Indicator. The steam and gas 
engine indicator is an extremely valuable instrument for engineer- 
ing purposes when used intelligently, but when in the hands of a 
careless inexperienced operator the results obtained may be little 
short of worthless. The instruments constructed by reputable 
manufacturers are reliable for the purposes for which they were 
intended and are indispensable in a steam or gas engine power 
plant of any considerable size. For scientific and investigative 
purposes the most reliable instrument should be used and the 
operator should be careful and experienced, in order that the best 
possible results may be obtained. Many operators make use of 
the indicator with a desire to secure reliable information and in 
many instances are sincere and painstaking in their efforts, but, 
unless they give proper attention to certain fundamental precau- 
tionary rules, the accuracy of the results secured may be questioned. 

Attachment of Indicator. Short Connections Desirable. As 
has been previously stated, in order to secure reliable diagrams, 
the indicator should be attached as close to the cylinder as con- 
ditions of the particular installation will permit. Long pipe connec- 



STEAM ENGINE INDICATORS 85 

tions result in unreliable indications. Generally speaking, other 
conditions remaining the same, the shorter the connections the 
more accurate the results. Most modern steam engines are now 
made with suitable holes which are tapped for indicator connec- 
tions. When the cylinders are not drilled and properly tapped 
for receiving the indicator, the engineer in charge should be com- 
petent to do it under the directions here given. 

Conditions Affecting Location of Indicator. Before deciding 
just where the holes should be drilled, it is desirable that all con- 
ditions of the case be carefully studied with a view of devising 
the whole plan for indicating the engine. It usually happens that 
the reducing motion, or drum motion as it is sometimes called, 
can be erected more advantageously in one position relative to the 
engine than another, or one kind may be better adapted for a 
given place than another. The type of engine, location Qf the 
steam chest or valves, the kind of cross-head and the best means 
of attaching to it, and the position of the eccentric, its rods, and 
connections, all should be given careful consideration in determining 
the best places to locate, the indicator. A free passage for steam 
to the indicator is of prime necessity and a location of the indicator 
insuring convenience in operation is desirable. The instrument 
can be used in a horizontal position but in taking diagrams it is 
more convenient when in a vertical position. Then again, the 
vertical position is that in which it would most probably be 
calibrated and for this reason alone is preferable. A prominent 
manufacturer gives the following directions for drilling cylinders 
to receive indicators: 

Mounting Indicator on Cylinder. When drilling holes in the cylinder the 
heads should be removed if convenient, so that one may know the exact posi- 
tion and the size of the ports and passages and be able to remove every chip 
or particle of grit which might otherwise do harm in the cylinder or be car- 
ried into the indicator and injure it. When the heads cannot be taken off, 
it can be arranged so that a little steam may be let into the cylinder when 
the drill has nearly penetrated its shell, so that the chips may be blown 
outward — care being taken not to scald the operator. 

It is essential that the holes be drilled into the clearance space at points 
beyond the travel of the piston so as not in any way to obstruct the passage 
of steam to the indicator. The most common practice in the case of hori- 
zontal engines is to drill and tap the holes in the side of the cylinder at each 
end. On certain types of horizontal engines, it is possible to drill and tap 
into the top of the cylinder at each end, in which case the indicator cocks can 



86 STEAM ENGINE INDICATORS 

be screwed directly into the holes. On vertical engines, the upper indicator 
is frequently connected into the cylinder head, although better results will be 
obtained if both holes are drilled and tapped in the side of the cylinder. 

Reducing Motions. It sometimes happens that in an effort 
to get quick results a makeshift type of reducing motion, or drum 
motion, is resorted to, with the almost inevitable result that the 
diagrams secured by its use are extremely faulty and in some cases 
worthless. In the long run, the most satisfactory results are 
secured if some form of approved reducing motion, such as has 
already been described, is used. Results of experience have shown 
that diagrams varying in lengths from 2J to 3| inches, depend- 
ing upon the speed of the engine, are most satisfactory. These 
lengths have been found long enough to admit of all useful divi- 
sions, and the movement of the card is slower and the tracing 
correspondingly more delicate and accurate than if a longer card 
is made. These facts should be borne in mind in designing and 
proportioning the reducing motion. 

Drum Spring Tension. A great many operators give no 
attention to the tension of the drum spring, using the same adjust- 
ment for testing engines operating at wide ranges of speeds. 
Theoretically speaking, there is only one correct drum spring 
tension for one speed, other conditions remaining unchanged, 
but the refinement need not be carried to this point. However, 
it is a matter which should at least receive some attention. For a 
particular installation and speed the tension should be a sufficient 
amount, and no more, to overcome the friction of the pencil on 
the paper and maintain, at all times, the indicator cord taut. 
Any very great amount of tension, in addition to that necessary, 
not only affects the wearing qualities of the instrument but shortens 
the life of the indicator cord. 

Adjustment of Guide Pulley. As has been heretofore 
explained, the object of the guide pulley is to properly conduct 
the indicator cord from the drum to the reducing motion. It is 
such an insignificant piece of mechanism that it is frequently 
overlooked by the inexperienced operator. Whenever this occurs, 
the diagrams are usually unsatisfactory in many respects, as can 
readily be seen, and in a very short time there results a broken 
indicator cord which, under certain conditions, is extremely 



STEAM ENGINE INDICATORS 87 

difficult . to repair. The adjustment of the guide pulley is one of 
the first adjustments which should be made in setting the indi- 
cator for taking cards. 

Adjustment of Pencil Pressure. In the early forms of indi- 
cators, the diagram was drawn on plain paper by means of a 
graphite pencil, the pencil being sharpened to a fine round point 
by means of a knife or fine file. The graphite pencil can still be 
used but a more satisfactory result is obtained by the use of a 
brass point for a pencil in connection with chemically prepared 
paper, known as metallic paper. In either case the result desired 
is the securing of a light distinct diagram, that is, a diagram 
which is distinct yet made by using a pencil pressure no greater 
than is absolutely necessary. This is a matter that is quite gen- 
erally overlooked by the average operator, the tendency being to 
obtain a diagram showing much contrast. The operator should 
always remember that within certain limits the lighter the line 
the more accurate the results. If too great a pressure is employed 
between the pencil and paper, the pencil will lag on both ascending 
and descending pressures, with the result that the diagram will be 
too small and will not represent the true power of the engine. 
It will not only give an incorrect indication of the power and 
pressures but also will improperly represent the true location of 
the various events of the cycle. 

Miscellaneous Precautions. Importance of Rules for Use of 
Indicator. It should be the sincere effort of every operator to 
secure the very best results possible when making use of the 
indicator. To this end very careful attention should be given 
to the brief rules prepared by the manufacturers for assembling 
and manipulating the instrument. At least attention should be 
given to these general directions until one becomes thoroughly 
familiar with the apparatus. For example, in the case of the 
Crosby indicator, the directions given on pages 30 to 33 of this 
text, inclusive, should be thoroughly digested and mastered. No 
matter where the indicator is made or by whom, the operator 
should adopt a correct and regular method of procedure in its use 
so that it becomes a habit. 

Care in Handling Indicator. Perhaps one of the chief reasons 
that the indicator receives so many damaging knocks and blows is. 



88 STEAM ENGINE INDICATORS 

the manner in which it is removed . from its carrying case and 
assembled. On opening the carrying case preparatory to taking 
diagrams, the indicator should at once be lifted out and attached to 
the indicator cock where it will be securely held while the spring 
and parts are being connected and adjusted. Before attaching 
the indicator, however, the cock should be opened for a very brief 
time and steam be permitted to blow through so as to blow out 
any foreign matter which might be detrimental to the correct 
action of the instrument. When the indicator is not in use, it is 
preferable to place a cap on the indicator cock, which cap is 
usually furnished with the indicator. 

In lifting the indicator from one position to another, never 
do so by taking hold of the drum as many instruments are ren- 
dered useless by carelessness in this regard. In some designs the 
drum is not held in position by means of a small thumb nut but 
slips off very easily. 

Lubrication. The question of proper lubrication is one which 
should be handled intelligently. Always before placing the piston 
and spring in the proper working position, the piston should 
receive a generous supply of oil. For steam engine work a good 
grade of valve oil should be employed, while for gas engine work 
a good quality of gas engine oil should be used; machine oil should 
never be used. For air compressor work and hydraulic work a 
high grade of light oil should be used. Occasionally the pencil 
mechanism should receive oil, which should be light and offer 
little tendency to cause gumming. Cases are on record in which 
diagrams were taken with the indicator piston lubricated with the 
wrong kind of oil and much time was spent and trouble experi- 
enced in an effort to diagnose an apparent error which did not 
exist. Hence the necessity for proper lubrication. 

Causes of Incorrect Indication. In taking diagrams many 
things may happen which will result in incorrect indication. In 
taking a series of cards, if one notices a card which is much shorter 
than all others, it may be due to one of two things: either the 
indicator cord is not properly connected to the reducing motion 
or by some means it has become too long. 

It will sometimes happen that the pressures on one card are 
much smaller than on others in the series. When this occurs, if 



STEAM ENGINE INDICATORS 89 

there has been no material change in steam pressure, it is very 
probably caused by the indicator cock being opened only partially. 
A leaky indicator cock is always a source of much annoyance. It 
not only causes incorrect indications of pressures, especially on the 
exhaust side of the diagram, but produces an irregular atmospheric 
line which otherwise would be straight. 

A diagram which shows an abnormal back pressure, when the 
engine is known to have but very little back pressure, is most 
probably due to a loosely connected piston or pencil mechanism, 
unless it is caused by a sticky piston. In the latter case, however, 
other indications on th^ diagram would probably reveal the facts. 

Modifications of Indicator for High Speeds. The indicator as 
usually constructed will give satisfactory results for all ordinary 
speeds. It cannot be used successfully for speeds above 450 
revolutions per minute. For the higher speeds it is necessary to 
use a heavier spring than would be needed for the same pressure 
at lower speeds. For such high speeds it is also desirable to use a 
reducing motion proportioned so as to give a card having a length 
not to exceed 2 or 2J inches. Best results are secured when 
the indicator is used at speeds below 200 revolutions per minute. 




31 



en 


» 


en 




H* 




J 


ca 


Pi 


3 


o 


-c 


o 


£ 


X 




H 


3j 


P 


_ 


P 


Si 

s 


>< 
> 
< 


a 
| 


W 


c 


W 


V 


P 


s 


S5 


u 


P 

O 


5 
S 


a* 


-Q 


S 


e 


o 


a 


o 




en 


~ 


en 


~ 


O 


£ 




& 




3 


o 


<r. 


rt 


8. 


& 


•- 


PQ 




00 


~~ 




•—, 


s 


© 


« 


a 


<: 


2 


W 


t 

3 


6 


O 


& 


^ 


9 




w 




-J 




fe 





PART II 

VALVE GEARS 



VALVE CHARACTERISTICS 

Function, Steam enters the cylinder of a steam engine through 
ports which must, in some manner, be opened and closed alter- 
nately, in order to admit and exhaust the steam at the proper time. 
To accomplish this purpose, one or more valves are moved back 
and forth across the port openings. A complete understanding of 
the valve and valve gear is essential to the engineer as well as to 
the designer, for even though a valve be properly designed, the 
economy of the engine may be seriously impaired by improper 
valve setting. The design and adjustment of these valves play a 
very important part in the efficient action of the steam engine. 

A valve gear is a mechanism consisting of a combination of 
slotted links, eccentrics, rods, levers, and other devices designed to 
operate valves of various types. The valve gear is separate and 
distinct from the valve. It operates the valve or valves but, 
strictly speaking, is not a part of them. This being true, one type 
of valve gear may be applied or used in connection with several 
different types of valves. For instance, the Stephenson gear may 
be used to operate a plain slide valve on one engine, a piston valve 
having either inside or outside admission on another, while a third 
may be attached to a more complicated form of valve mechanism. 
It should be borne in mind, therefore, that the valve gear is a 
separate and distinct part of the steam engine and that its func- 
tion is to impart motion to the valve or valves. 

The valves, in turn, perform the following functions during the 
engine cycle: 

(1) Admission. This begins when the valve opens to admit steam 

to the cylinder. 

(2) Cut-Off. This is the point at which the valve closes to cut 

off the admission of steam. 

(3) Expansion. This takes place from cut-off to release. 



VALVE GEARS 



(4) Release. This begins when the exhaust port is opened. 

(5) Compression. This begins when the exhaust port is closed. 
There may be a single valve to regulate admission and exhaust 

or there may be a double set of valves, one set to admit the steam 
at each end and another to release it. The valve may have a plain 
reciprocating motion, moved either by a rod or by some device that 
releases at the proper time, allowing the port to close suddenly under 
the influence of counterweights, springs, or vacuum dashpots. To 
the first class belong the plain slide valve and its modification of 
piston valve, gridiron valve, etc.; to the second class belong such 
valves as the Corliss, the Brown, and others. 

The simplest type of valve is the plain slide, or D, valve as shown 
in Fig. 1, in which V is the valve, R is the valve rod, K is the exhaust 
cavity, Pi and P 2 are the steam ports, E is the exhaust port, A B is 
the valve seat, and DM are the bridges of the valve seat. The 
valve seat must be planed perfectly smooth, so that steam pressure 
on the valve will make a steam-tight fit and cause as little friction 

as possible when the valve 
moves back and forth. 
Furthermore, the length of 
the seat A B must be a 
little less than the distance 
from the extreme right- 
hand position of the right- 
hand edge of the valve to 
the extreme left-hand posi- 
tion of the left-hand edge of the valve. This allows the valve at 
each stroke to slightly overtravel the seat, thus keeping it always 
worn perfectly flat and smooth. If the valve seat were not raised 
slightly above the rest of the casting, or if it were too short, the 
constant motion of the valve would soon wear a hollow path in the 
valve seat, and it would cease to be steam tight. 

Eccentric. The valve usually receives its motion from an 
eccentric, which is simply a disk keyed to the shaft in such a manner 
that the center of the disk and the center of the shaft do not coin- 
cide. It is evident that as the shaft revolves, the center of this 
eccentric disk moves in a circle about the shaft as a center, just as 
if it were at the end of a crank. The action of the eccentric is equiva- 




Fig. 1. Plain Slide, or D, valve 



VALVE GEARS 



lent to the action of a crank whose length is equal to the distance 
between the center of the eccentric and that of the shaft. 

Fig. 2 represents the essentials of an ordinary eccentric. Oi is 
the center of the shaft, 2 is the center of the eccentric disk E, and 
S is a collar encircling the eccentric and attached to the valve rod R. 
As the eccentric turns in the strap, the point 2 moves in the dotted 
circle around Oi and the point Ai also moves in a circle. When half 
a revolution is accomplished, the point 2 will be at 3 , the point Ai 
will be at \A 2 , and the eccentric strap and valve rod will be in the 
position indicated by the dotted lines. The distance X 2 of the 




3/ ! i R 

3' /'/ L.-d 



---- - 7 ' 



/,- 






Fig. 2. Details of Ordinary Eccentric 



center of the eccentric from the center of the shaft is known as the 
eccentricity, or throw, of the eccentric. The travel of the valve is 
twice the eccentricity. 

Since the eccentric transmits the motion of the revolving shaft 
to the valve, it will be necessary to study the relative motions of 
crank and eccentric in order to obtain a clear idea of the steam dis- 
tribution. This relation will be developed in connection with the 
discussion of the valve action which follows. 

Valve Motion. Valve without Lap. Fig. 3 shows a section 
through the steam and exhaust ports of an engine, together with a 
plain slide valve placed in mid-position* and so constructed that 



* A valve is in mid-position when the center line of the valve coincides with the cen- 
ter line of the exhaust port. 



VALVE GEARS 



in this position it Just covers the steam ports. Referring to Fig. 1, 
which shows the same type of valve drawn to a larger scale, suppose 
the valve is moved a slight distance to the right; the port Pi is then 





-4--' 



Fig. 3. Cylinder Details_Showing Plain Slide Valve without Lap in Mid-Position 

uncovered and opened to the live steam which enters the cylinder 
and causes the piston to move. Since the two faces of the valve are 
just sufficient to cover the steam ports, it is evident that as the 
port Pi opens to live steam, the port P 2 opens to the exhaust. The 
ports are closed only when the valve is in mid-position. This allows 
admission and exhaust to continue during the whole stroke. With 
such a valve, there is no expansion or compression; the indicator 
card is a rectangle, and the m. e. p. is equal to the initial steam 
pressure, assuming no frictional losses in the steam pipe or conden- 
sation in the cylinder. 

For a theoretical discussion of valve motion, it is assumed that 
the eccentric rod moves back and forth in a line parallel to the center 







<iy 



Fig. 4. Position of Piston and Valve in Cylinder Shown in Fig. 3, after One-Half Stroke 

line of the engine. This is not the case in practice, for the eccentric 
rod always makes a small angle with the center line, just as the con- 
necting rod does, but the eccentricity is so small in comparison with 



VALVE GEARS 5 

the length of the eccentric rod that the angularity of the eccentric 
rod is very much less than the angularity of the connecting rod, and 
its influence may be neglected without appreciable error. 

When the valve shown in Fig. 3 is in mid-position, the crank 
is on dead center, the eccentric is set at right angles to it, and the 
piston is just ready to begin the stroke. 

Fig. 4 shows the relative positions of the crank A, piston B, 
eccentric E, and valve V, when the crank has made a quarter turn 
or the piston has moved to about half -stroke. The eccentric is now 
in its extreme position to the right, the valve has its maximum dis- 




Fig. 5. Details of Cylinder, Showing Valve with Lap 



placement, and both the steam and exhaust ports are wide open. 
The valve will not close again until the. piston has reached the end 
of its stroke. 

This type of valve is used only on small engines and, since it 
allows no expansion of the steam, is very uneconomical. Further- 
more, it will be seen that this valve opens just after the stroke begins, 
which is impractical, for it means that the piston has begun its 
stroke before the full steam pressure reaches it, which will cause an 
inclined admission line on the indicator diagram. 

Valve with Lap. If the face of the valve is made longer than 
shown in Fig. 1, so that in mid-position it overlaps the steam ports, 
we shall have a valve such as shown in Fig. 5. The amount that 



6 VALVE GEARS 

the valve overlaps the steam ports when in mid-position is called 
the lap of the valve. In Fig. 5, D I is the inside lap and C is the out- 
side lap. 

It will at once be seen that both the admission and exhaust ports 
may remain closed during a part of the stroke, thus making expan- 




se 




Vi. 



Fig. 



Valve with Inside and Outside Lap Set for Admission 



sion and compression possible. It is also evident that steam can 
not be admitted until the valve uncovers the port by moving from 
mid-position a distance equal to C. Admission continues until 
the valve returns to such a position that the outer edge of the valve 
again closes the port. Release will begin when the inner edge of 
the valve begins to uncover the port. 

Analysis of Motion. Fig. 6 represents a valve, having both 
inside and outside lap, which is set at the point of admission. Since 




Fig. 7. 



ve Set at Maximum Displacement 



the valve must move over a distance equal to the outside lap 
in order that admission may take place under proper conditions, it 
is evident that the eccentric can no longer be at right angles to the 
crank at the beginning of the stroke, but must be in advance of the 



VALVE GEARS 



right-angle point by an amount equal to the- angle EO C, known as 
the angular advance. 

The maximum displacement of the valve is attained when the 
eccentric is horizontal, as shown in Fig. 7. In this position, both 
the steam and the exhaust ports are wide open, and any further 




-4- 



Fig. 8. Valve Position with Steam Port Closed on Head End 

motion of the piston will cause the valve to move toward its mid- 
position. 

Admission continues until the valve returns to the position 
shown in Fig. 8. Here the outside lap just closes the left-hand steam 
port, cut-off takes place, and the steam already in the cylinder begins 
to expand. As the valve continues to move toward the left, the 
left-hand inside lap begins to uncover the left-hand port and releases 
the steam at the position shown in Fig. 10. The dotted lines of 
Fig. 7 show the valve in its extreme position to the left, while the 




Pig. 9. Valve Position with Exhaust Port Closed on Crank End 

dotted position of crank and eccentric in Fig. 10 shows the valve 
returned to the point of compression, which continues until the con- 
ditions of Fig. 6 are again reached and the opening valve allows steam 
again to enter the cylinder. 



8 VALVE GEARS 

This process has been traced step by step for one end only; let 
us now consider what is happening at the other end. While the 
crank A is moving from the position shown in Fig. 6 to that in Fig. 8, 
steam is being admitted to the head end and being exhausted from 




>/ --• 

Fig. 10. Position of Valve and Cylinder for Head-End Release 

the crank end. As the inside lap is less than the outside lap, the 
exhaust continues longer than the admission. 

Fig. 9 shows the relative positions of crank, eccentric, and 
valve when the exhaust closes on the crank end and compression 
begins. Between these two positions, the steam is expanding in 
the head end and exhausting from the crank end. Between the 
positions of Figs. 9 and 10, both ports are entirely closed, and expan- 
sion is taking place in the head end and compression in the crank 
end. In Fig. 10 is shown the position of the valve for head-end 




Fig. 11. Position of Valve and Cylinder for Crank-End Admission 

release. Fig. 11 shows admission at the crank end of the cylinder 
and marks the end of crank-end compression. 

Effect of Change of Lap. By referring to Figs. 6 to 11, the effect 
of any change of lap may at once be observed. If the outside lap 
is increased, the valve must move farther from mid-position before 



VALVE GEARS 



9 



admission will occur and on the return, after the maximum displace- 
ment is reached, the greater outside lap will close the port sooner, 
and the point of cut-off shown in Fig. 8 will be reached before the 
crank reaches the angle there shown. A decrease of outside lap will 
make cut-off later and admission earlier. 

On the other hand, if the inside lap is increased, the valve must 
move farther before release occurs and the crank angle will be greater 
than that shown in Fig. 10, while on the return to the dotted position, 
the port will close earlier and make an earlier compression. The 
crank angle will be less than is there shown. Decreasing the inside 
lap will cause earlier release and later compression. 

Thus we see that it is the outside lap that influences admission 
and cut-off, and the inside lap that controls release and compression. 
For this reason the outside lap is often called the steam lap and the 
inside lap is called the ex- 
haust lap. 

Lead. If a valve hav- 
ing lap is in mid-position, 
the port is closed and the 
engine can not start, because 
no steam can enter the cyl- 
inder. That the steam may 
be ready to enter the cylin- 
der at the beginning of the stroke, it is necessary that the eccentric 
be set more than 90 degrees ahead of the crank as already men- 
tioned, thus making the eccentric radius take an angular advance 
EOC, as shown in Fig. 6. In order that the ports and all clearance 
space may be properly filled with steam at the beginning of the 
stroke, it is necessary that the valve be displaced from its mid-posi- 
tion an amount slightly greater than the outside lap. With the 
piston at the end of the stroke, the valve will have a position as shown 
in Fig. 12, the port being open the distance A B, the lead of the valve. 
This causes the eccentric to be moved forward a slight amount in 
excess of the lap angle. This excess is called the angle of lead. 

In Fig. 13, O2R2 represents the position of the crank at the begin- 
ning of the stroke, X0i.4i the lap angle, and A1O1A2 the angle of 
lead. The eccentric, to give lead, -must be set at the angle R1O1A2 
ahead of the crank or 90 degrees plus the angular advance. In large 




Fig. 12. Position of Valve Showing Lead 



10 



VALVE GEARS 



high-speed engines, a liberal lead is essential in order that the ports 
and clearance space may be well filled with steam before the stroke 
begins. If there is no lead, a portion of the steam will be used in 

filling these places and full steam 
pressure will not reach the piston 
until it is well advanced on the 
stroke. This will give a sloping 
admission line, as shown in Fig. 
14. Too much lead, on the 
other hand, will cause too early 
an admission, as shown in Fig. 
15. 

If the angular advance is in- 
creased, the eccentric will be 
moved further ahead of the 
crank, and consequently it will 
arrive at each of the events 
sooner than before. If, then, 
the angular advance is increased, 
all of the events of the stroke will occur earlier. 

Effect of Lead. From the foregoing discussion of lead, it is 
evident that its effect is to permit steam to enter the cylinder before 
the end of the stroke, which tends to provide an abundance of steam 
behind the piston when starting the return stroke and throughout 
the period of admission. It also promotes smooth running of the 
engine by furnishing a cushion or retarding force to the moving 
parts, thereby eliminating the "knocks" or "pounds" incident to 
lost motion. Since the effect of lost motion depends upon the weight 




Fig. 13. 



Diagram Showing Lap Angle and 
Angle of Lead 




Fig. 14. 



Indicator Diagram Showing 
Effect of No Lead 



Fig. 15. Indicator Diagram Showing 
Too Early Admission 



and velocity of the reciprocating parts, it is evident that the amount 
of lead required will vary for different engines and for the same engine 
running at different speeds. The exact amount of lead can not be 
determined except by trial and by use of the steam engine indicator. 



VALVE GEARS ■ 11 

When experimenting for the determination of the proper amount 
of lead for a specific case, it will be necessary to gradually increase 
the angular advance until smooth running is obtained. After this 
result is obtained, indicator cards should be taken to see if the lead 
is excessive, in which case the valve must be adjusted until the desired 
conditions are obtained. Since lead permits steam to act against 
the piston before the end of the stroke, it results in negative work, 
hence the amount of lead should not be excessive. An amount of 
lead sufficient to insure the filling of the clearance space is permissible, 
but very much more than this is detrimental to the economic per- 
formance of the engine. 

Analytical Summary of Valve Terms. Thus far in discussing 
the plain slide valve, a number of terms have been used that are of 
primary importance and must be thoroughly understood in order 
to properly grasp much that is yet to be studied. It seems advis- 
able, therefore, that a recapitulation of the terms used be presented. 

Mid-Position. A valve is said to be in mid-position when the 
center of the valve and valve seat coincide. When in this position, 
the steam ports are all closed. 

Displacement. The displacement 01 a valve is the amount the 
valve has been moved either to the right or left of its mid-position. 
In Fig. 4, the valve has moved to the right a distance equal to the 
width of the steam port, hence in this instance the displacement of 
the valve is equal to the width of the steam port. 

Valve Travel. The travel of the valve is the distance the valve 
travels in moving from one extreme position to the other. The 
travel of the valve is twice the eccentricity, or throw of the eccentric. 

Eccentricity. The eccentricity, or throw of the eccentric, is 
the distance between the center of the shaft and the center of the 
eccentric. It is equivalent to a crank, the length of which is one- 
half the valve travel. For instance, if the valve travel of an engine 
is 6 inches, the eccentricity, or throw of the eccentric, would be 3 
inches, or one-half of the valve travel. 

Lap. The amount that the valve extends over the steam port 
when in mid-position is called steam lap or often spoken of as the 
lap of the valve. The steam lap is equal to C in Fig. 5. In Fig. 5, 
it is obvious that when the valve is in mid-position, the distance 
D I is called exhaust lap. Steam lap and exhaust lap are frequently 



12 VALVE GEARS 

spoken of as outside and inside lap, respectively. The effect of the 
exhaust lap is to delay exhaust and hasten compression. 

Very frequently a valve does not have any exhaust lap and 
there is a small port opening between the cylinder and the exhaust 
cavity when the valve is in mid-position, as shown at A, Fig. 19.' 
In such a case, the valve is said to have inside clearance. The effect 
of inside clearance is opposite to that of exhaust lap, namely, it delays 
compression and hastens exhaust, and insures a minimum amount 
of back pressure. 

Lead. By the term lead is meant Lne amount the steam port is 
open when the engine is on either dead center. 

Angle of Advance. It was noted in Fig. 1 that the crank and 
eccentric were exactly 90 degrees apart and that admission occurred 
at the beginning and cut-off at the end of the stroke. On account 
of economic reasons, this is not a good arrangement. Hence we find 
that the valves on all engines have lap and are set to give the neces- 
sary amount of lead. In order to obtain lead when the engine is on 
dead center with a valve having lap, it is necessary to turn the eccen- 
tric ahead, in the direction the engine is to run, through such an 
angle that the valve will be displaced by an amount equal to the lap 
plus the lead. The angle measuring this displacement is the sum of 
the angle of lap and the angle of lead. If there is no lead, this angle 
would be decreased by the angle of lead. The sum of the angle of 
lap and the angle of lead is frequently designated as the angle of 
advance. The angularity between the eccentric and the crank then 
becomes equal to 90 degrees, plus or minus the angle of advance 
according to the type of valve and gear. 

Inequality of Steam Distribution. In the valve diagrams thus 
far considered, the events of the stroke have been discussed for each 
end separately, without reference to the relation of similar events 
on the other side of the piston. If the connecting rod were of infinite 
length, so that it would always remain parallel co the center line of 
the engine, the distribution would be the same for both ends of the 
cylinder. In practice, the connecting rod varies from four to eight 
times the length of the crank, which causes the connecting rod 
always to be at an angle to the center line of the engine when the 
engine is off dead center, and for a given crank angle makes the piston 
displacement greater at the head end than at the crank end. 



VALVE GEARS 



13 



To Find Displacement of Valve. The circle, Fig. 16, represents 
the path of the eccentric center during a complete revolution of the 
engine. C represents the crank, and R the corresponding posi- 
tion of the eccentric. The diam- 
eter X Y represents the extent 
of the valve travel. Since the 
eccentric rod is so long in com- 
parison to the eccentricity, we 
make no appreciable error by 
assuming it always to be parallel 
to the center line of the engine. 
When the eccentric is at OL, 
the valve is in mid-position. At 
R the valve has moved from 
mid-position an amount ON, 
found by dropping a perpendic- 
ular from R to the center line 

X Y. If the angularity of the connecting rod could be neglected, 
the piston displacement could be found in the same manner. 

To Find Displacement of Piston. To find the displacement of 
the piston, a diagram as shown in Fig. 17 must be drawn. In this 
figure, A B represents the cylinder, Pi the piston, Hx the crosshead, 
II iR the connecting rod, and R the crank. Suppose now the engine 
should stop and the piston be clamped in this position. The piston 
displacement would be represented by A Pi. If the crank pin at R 
should now be loosened so as to allow the connecting rod to fall to a 




Fig. 16 



Eccentric Circle Showing Relative 
Positions of Crank and Eccentric 



1 






\ 












1 




p, 


% 


i 
i 


4 


/£ 


1 






i 






tfw- 



Fig. 17. Diagram for Finding Displacement of Piston 



horizontal position, the point R would describe the arc of a circle 
RN, and X N would represent the piston displacement and would 
be equal to A P x . Suppose now that in this disconnected way, the 



14 VALVE GEARS 

piston, crosshead, and connecting rod were moved forward until the 
end of the rod came to 0. Pi would then be at P 2 and the piston 
would be in the middle of its stroke. Now suppose the end of the 
rod were swung up to its proper position on the crank-pin circle, the 
piston remaining stationary. The end of the rod would describe an 
arc OZ; the crank pin would be at Z, less than a quarter revolution 
from X; while the piston would be in the middle of its stroke. 

Suppose this engine were running with cut-off at half stroke on 
the head end, and that XOZ represented the corresponding crank 
angle. On the return stroke, the valve would cut off at the same 
crank angle YOT, which is equal to XOZ, and T would represent 
the crank position for cut-off on the return, or crank-end, stroke. 
The piston, as we have just seen, will not be at half stroke except 



Fig. 18. Crank and Eccentric Diagram for Engine Shown in Fig. 17 

when the crank is at OZ or OS. Consequently, the crank position 
T is less than half stroke and cut-off occurs earlier at crank end 
than at head end. When the crank is at OZ, the eccentric will be 
at OA i, Fig. 18, assuming the valve to have no lap, and the valve 
displacement will be Bi. When the crank is at T, the eccentric 
will be at 0A 2 and the valve displacement will be B 2 , which is equal 
to OB i t the displacement of the valve at cut-off on the head end. 
The piston displacement will be OX on the head end and W Y on 
the crank end when cut-off occurs. If the connecting rod always 
remained parallel to the center line, the cut-off would be the same 
at both ends. 

Compensation of Cut-Off. It has been pointed out that length- 
ening the outside lap makes the cut-off earlier, and that shortening 



VALVE GEARS 



15 



the lap makes it later. The cut-off in the case just cited may then 
be equalized by altering the outside laps. If we increase the out- 
side lap on the head end, or decrease the crank-end lap, the inequality 
will be less. By changing either or both of the laps the proper 
amount, the cut-off may be exactly equalized. 

But altering the outside lap changes the lead, as has already 
been explained. If the lap is increased on the head end, the lead 
will be less than on the crank end. If the lead becomes too small 
on the head end, the angular advance may be increased but the 
inequality of lead will still remain, for this increase of angular advance 
will increase the lead at the crank end as well as at the head end, and 
by hastening all the events of the stroke may give a bad steam dis- 
tribution if the proper care is not taken. 

Unequal lead is of less consequence on a low-speed engine than 
on a high-speed engine. On low-speed engines, the cut-off may be 




Fig. 19. Valve in Mid-Position Showing Inside Clearence, or Negative Lap 



equalized at the expense of lead with beneficial results, but on high- 
speed engines, this is not true. A high-speed engine requires more 
lead than a low-speed engine, for there is relatively less time in each 
stroke for the clearance space to fill with steam. 

If both inside laps are equal, compression will not occur equally 
at both ends. To equalize it, the inside laps may be changed in the 
same manner as the outside laps are changed to equalize the cut-off. 
By altering these inside laps to equalize compression, it may happen 
that the lap is reduced enough to leave the exhaust port open when 
the valve is in mid-position. The amount of this opening is called 
inside clearance, or negative lap. This is illustrated at A, Fig. 19* 



16 



VALVE GEARS 



Rocker. Sometimes it happens that the valve stem and eccen- 
tric rod can not be so placed that they will be in the same straight 
line; or it may be that the travel of the valve must be so great as to 
require an excessively large eccentric. In such cases, a rocker may 
be used. 




Fig. 20. Valve with Rocker Arrangement 

Fig. 20 shows a valve that is not in line with the eccentric. An 
instance where this occurs is in horizontal engines when the valve 
is set on top of the cylinder instead of on one side. By means of 
the rocker A G, the valve may receive its proper motion. 

In case it is more convenient to place the pivot of the rocker 
arm between the connections to the valve stem and those of the 
eccentric rod, such an arrangement as is shown in Fig. 21 may be 
used. Here it will be noticed that the valve stem and eccentric rod 
are moving in opposite directions and in order to give the valve the 
same motion as in Fig. 20, the eccentric must be moved 180 degrees 
ahead of the position there shown. 

If A B is less than A G, the valve travel will be greater than twice 
the eccentricity, in proportion as A G is greater than A B. In all 




K<? 



/ /" 



&)) 



Fig. 21. 



Arrangement of Rocker by which Valve Stem and Eccentric Rod Move 
in Opposite Directions 



cases, the valve travel is in the same proportion to twice the eccen- 
tricity as A G is to A B. Thus, if the valve travel is 4| inches, 
A B is 15 inches, and A G is 18 inches, then If X4§ = 3f inches, will 
equal twice the eccentricity. 

A valve gear may be so laid out as to make both the cut-off and 
the lead equal for both ends of the cylinder. This may be done by 



VALVE GEARS 



17 



a proper proportion between the rocker arms, and a careful location 
of the pivot of the rocker. The eccentric must then be set accord- 
ingly. In this manner, the Straight Line engine equalizes the cut-- 
off and lead. A discussion of this method will be considered later. 

VALVE DIAGRAMS 

Zeuner Diagrams. In order to study the movements of the 
valves, the effects of lap, lead, eccentricity, etc., diagrams of various 
sorts have been devised. By the use of diagrams we may acquire 



HEAD A 
END * 




Fig. 22. Zeuner Diagram for Valve Analysis 



a knowledge of the valve motion without the complex mathematical 
expressions that such a discussion would entail. The most useful 
of these various diagrams is that devised by Zeuner and, to avoid 
complexity, we shall confine ourselves to a discussion of this dia- 
gram alone. The eccentric rod is assumed to be of infinite length, 
and the positions of the crank are shown on the diagrams. The 
displacement of the piston can easily be found if the ratio of crank 
to connecting rod is known. 



18 VALVE GEARS 

The function of the Zeuner diagram is to show the relation 
between the valve positions and crank positions. This relation 
being known, it is a simple matter to obtain the eccentric and piston 
positions. 

In Fig. 22, AO E represents the valve travel, and the center 
of the eccentric will move in the circle AC EG. It is assumed, 
also, that this circle represents the path of the crank center, hence 
this circle will be known as the crank and valve circle. OA is the 
position of the crank and OH is the corresponding position of the 
eccentric, when the engine is on the head-end dead center. Since 
this valve has lap, and since admission must occur before the end 
of the stroke, it is evident that the eccentric must precede the crank 
by 90 degrees plus the angle of advance d. From H drop a per- 
pendicular line upon the center line AO E, thus locating the point Pi. 
The distance OPi is the amount the valve has been moved to the right 
of its mid-position when the crank is on dead center. Since the 
diagram gives the relation between crank and valve positions, the 
displacement of the valve Pi can be laid off from on the crank 
position OA, thus establishing the point P 2 . Turn the crank through 
an angle B to the position B. The eccentric will move through the 
same angle and will be found at I. Draw the perpendicular line 
IQ lt and OQi represents the displacement of the valve for the crank 
position OB. Lay off OQi on OB, establishing the point Q 2 . 
Continue the rotation of the crank until the point D is reached. 
The eccentric then will be found at E, and the valve will have its 
greatest displacement Ri to the right of its mid-position. It is 
evident that Ri is equal to D. If the rotation of the crank be 
continued in the direction of the arrow, the valve will return from 
its extreme position on the right and will approach its mid-position. 
By locating on the various crank positions the corresponding valve 
displacement, a series of points as P 2 , Q2, Ri, etc., will be obtained, 
all of which will lie on the circumference of a circle, as OP2Q2R2, 
the diameter D of which will make an angle d equal to the angle 
of advance laid off to the left of the vertical C. If this opera- 
ation be continued for a complete revolution, a series of points will 
be established in the lower quadrant, establishing a circle OPiF, 
the diameter of which will be a continuation of OD and, therefore, 
will make an angle d with the vertical but will lie on the right of 



VALVE GEARS 19 

the vertical line COG. These two circles are called valve circles, 
and they represent the movement of the valve to the right and left 
of its mid-position and, as previously stated, represent the amount 
the valve has moved for any crank position such as OB. 

Having established the valve circles, it is a simple matter to 
obtain the valve displacement for the position OB, which, in this 
case, would be the distance 0Q 2 cut off from OB by the valve circle. 
It can be proven that Q 2 is the valve displacement by comparing 
the two right triangles OIQi and ODQ 2 . They are equal because 



SO°& p 




Fig. 23. Diagram Showing Study of Valve Motion for Head End Only 

they are similar and have two corresponding sides OB and 01 equal. 
Therefore, 0Q 2 equals OQi. This being true for any crank position, 
it is true for all crank positions. 

Study of Valve Motion from Diagram. Now that the truth of our 
proposition has been proved, let us see how we may study the valve ' 
motion from such a diagram. In Fig. 23 let XY represent the 
valve travel; then the circle XEiFFwill represent the path of the 
center of the eccentric. Let d be the angle of advance and lay off 
EiO toward the crank, making an angle 6 with the vertical. Pro- 
duce Efi to F, and on E x and OF as diameters draw the valve circles 
as shown. With as a center and V, equal the outside lap, as a 



20 VALVE GEARS 

radius, draw an arc intersecting the upper valve circle at V and M. 
Lay off P equal to the inside lap and with as a center and P 
as a radius, draw an arc intersecting the valve circle at P and Q. 
Draw the crank-position line A passing through V. Then, when 
the crank is in this position, the displacement of the valve is equal 
to V (the outside lap) and the steam is ready to enter the cylinder. 
This is the position of the crank at admission, and the crank angle 
XOA is called the lead angle. The valve has lead and, therefore, 
the admission takes place before the end of the stroke. When the 
crank reaches the position 0E U the displacement of the valve is 
equal to the eccentricity OEi, and is at a maximum. Further 
motion of the piston causes the valve to move toward mid-position 
until, at the crank position OC, the displacement OM is again equal 
to the outside lap and the valve has reached the point of cut-off. 
When the position OHi is reached, the crank line is tangent to 
both valve circles and there is no displacement of the valve. At 
this point, the valve is in mid-position. 

Further crank movement draws the inside lap toward the edge 
of the exhaust port until, at the crank position B, the displacement 
is equal to OP (the inside lap) and release begins. Pit OF the maxi- 
mum valve displacement is again reached and the valve moves in 
the opposite direction until at OD its displacement from mid-posi- 
tion is again equal to Q, equals OP the inside lap, and compression 
takes place. At 0H 2 the valve is again in mid-position. At OX 
the displacement of the valve is 01, but since the valve has to move 
the distance OJ before the port begins to open, IJ must represent 
the port opening when the crank is on dead center, and by defini- 
tion we know that lead is the amount of port opening at this posi- 
tion. Therefore, IJ represents the lead. 

At the position R, the port is open an amount equal to TG; 
at Ei the opening is a maximum equal to EiN; at C the port is on 
the point of closing and there is no opening. If LW represents 
the total width of the steam port, the exhaust port will be open wide 
when the displacement of the valve is equal to W and it will remain 
wide open while the' . swings fiom OW to OK. 

If the width of' steam port in addition to the outside lap were 
laid off on the other valve circle, it would fall at E 2 . For the admis- 
sion port to be wide open, the displacement of the valve would have 



VALVE GEARS 



21 



to be equal to 0E 2 which is more than the maximum displacement. 
This shows that in this case the steam port is never fully open and 
that the left-hand edge of the valve overlaps the right-hand edge 
of the port by an amount equal to EiE 2 when the valve has reached 
its maximum displacement. 

Fig. 23, with its two valve circles, shows the diagram for the 
head end of the cylinder only. The crank-end diagram would be 
similar except that the laps might not be equal to those of the head 
end. 

Properties of Zeuner Diagrams. The Zeuner diagram deals with 
admission, cut-off, release, compression, lead, valve travel, angle of 



CUT-OFF HEADEND 



COMPRESSION 
CRANK END 




x f\RELEASE 
^^ HEAD END 



\ADA//SS/ON 
CRANK END 



HEAD 
END 



RELEASE , 
CRANK END \$ 



COMPBE3SJON 
HEAD END 



CRANK END 



CUT-OFF 
CFiANK END 



Fig. 24. 



Diagram Analysis for Movement of a Direct Valve as Regards 
Head End of Cylinder 



advance, maximum and minimum port opening, steam lap, and 
exhaust lap. Generally, if four of these be given, the others can be 
found. It is evident, therefore, that there are a great many possi- 
ble combinations, hence it is necessary r e definitely in mind 
and clearly understood the properties of the Ztuner diagram. The 
proofs given are for the movement of a direct valve as regards the 
head end of the cylinder. AH letters refer to Fig. 24. 



22 VALVE GEARS 

(1) The figure is symmetrical on the line BE. In the semi- 
circles OLB and OMB, OL equals OM, each being the radius of 
the steam-lap circle. Since OL equals OM, the arcs which they 
subtend are equal, therefore, the arcs LJB and MB are equal. 
This makes the angles LOB and MOB equal because they are 
measured by equal arcs. Therefore, BO bisects the angle LOM, 
and in a similar way it can be proved that E bisects the angle 
NOP. 

(2) The line B M is perpendicular to MR and is tangent to 
the steam-lap circle. The angle BMO is a right angle because it 
is inscribed in a semicircle. Therefore, B M is tangent to the steam- 
lap circle and is perpendicular to the crank position OMR. 

(3) The line joining the admission and cut-off points for the 
head end is perpendicular to BO and is tangent to the steam-lap 
circle. 

The triangle QOR is an isosceles triangle and, as demonstrated 
above, BO bisects the angle QOR, hence BO is perpendicular to 
the baseQR. To prove that QR is tangent to the steam-lap circle, 
it is necessary to show that the distance OH measured on BO is 
equal to OM, the radius of the steam-lap circle. The right triangles 
BOM and HOR are equal, having two sides equal and one com- 
mon angle. Hence, H is equal to M. 

(4) The line B J is perpendicular to AO. The angle B JO is 
a right angle, being inscribed in a semicircle. 

(5) The radius of the circle A I with center at A and tangent 
to QR, is equal to the lead JK. 

From the center A draw A G parallel to IH. In the right tri- 
angles BJO and AGO, the angle AGO equals B JO, being right 
angles. BO equals AO. The angle A OH is common to both 
triangles, therefore, they are equal. Hence, OJ equals OG. But 
OK equals OH. Therefore, GH equals JK, equals A I, which is 
the lead. 

By using Fig. 24 at all times as a reference figure and bearing 
in mind the things it tells, no great difficulty should be encountered 
in solving problems. To illustrate the principles set forth above 
and to give an idea of the practical use of the Zeuner diagram, sev- 
eral problems will be worked out as an indication of what may be 
done. 



VALVE GEARS 23 

ILLUSTRATIVE PROBLEMS 

In designing a slide valve, a few of these variables depend upon 
the conditions under which the engine is to run. For instance, the 
valve travel is limited, cut-off must be at a certain point, and the 
engine must have a certain lead. Then, with the aid of a Zeuner 
diagram, the remaining proportions of the valve may be determined. 

Example 1. Given a valve travel of 3 inches, exhaust lap of f inch, 
angular advance of 35 degrees, and crank angle at cut-off of 115 degrees. 
Determine the laps, the lead, and the crank angles at admission, compression, 
and release. 

Solution. In Fig. 25, let X Y represent the valve travel of 3 inches. 
Draw M perpendicular to X F, and on X Y as a diameter draw the circle 




F 

& 

Fig. 25. Zeuner Diagram for Finding Laps, Lead, and Crank Angles 

X M Y F representing the path of the center of the eccentric as it revolves 
about the shaft. Lay off the angle M E to represent the angular advance 
of 35 degrees so that the angle X E is equal to 90 degrees minus the angular 
advance. Produce E to F. Then on E and F as diameters, draw the 
valve circles. The eccentricity E or F, if no rocker is used, will be half 
the valve travel. Lay off the angle X C to represent the crank angle at 
cut-off of 115 degrees, and K will then represent the distance of the valve 
from mid-position when cut-off takes place. This distance we know is the 
outside lap. Draw the arc K I, known as the lap circle, and it will cut the 
valve circle again at V. When the valve is again the distance V, the out- 



24 VALVE GEARS 

side lap from mid-position, admission will take place. Draw the line OVA 
and this will represent the position of the crank at admission. 

When the crank is at O X, the valve displacement is equal to J. This 
is at dead center, and the valve is open the amount I J, for it has moved this 
distance more than the outside lap. Therefore, TV is the lead for this end. 

Now on the other valve circle, draw the arc P Q with the inside lap 
(f inch) as a radius. It will cut the valve circle at P and Q. When the valve 
displacement is equal to Q, the exhaust port has just commenced to open, 
and the engine is at release. In the same way, when the valve displacement 
is equal to P, the port begins to close and the engine is at compression. 
Q D represents the crank position at release and P B the crank position 
at compression. 

The results are tabulated as follows: 

Outside lap OK = f inch 

Angle of lead X O A = 5 degrees 

Linear lead I J = -^ inch 

Max. port opening for admission HE = f inch 

Crank angle at release X D = 185 degrees 

Crank angle at compression X B = 65 degrees 

Max. port opening for exhaust FN = f inch 

Fig. 25 is drawn full size, and all of the above measurements may readily 
be verified. This figure is drawn for the head end only. If the crank angle 
at cat-off is the same on both ends, the Zeuner diagram for the crank end will 
be exactly like Fig. 25. 

Example 2. Given a lead ye inch, valve travel 3 inches, steam lap 

r> 

(h.e. andc. e.) f inch, exhaust lap (h. e. and c.e.) fg inch. Let — — , that is, the 

Li 

ratio of the length of the crank to the connecting rod, equal h Construct the 
Zeuner diagram and find all the events for both the head and crank ends in 
per cents. 

Solution. Construct the valve travel circle A C D F, Fig. 26, with a 
radius of 1^ inches; the steam-lap circle with a radius H of f inch; and the 
exhaust lap circle with a radius R of ye inch. The steam-lap circle cuts the 
crank position for h. e. dead center at the point K. From K lay off the dis- 
tance J K to represent the lead of ye inch. At A, construct the lead circle 
with a radius of ye inch. From the properties of the Zeuner, we know that 
where a perpendicular erected at the lead point J cuts the valve travel circle 
as at B, the line B is the diameter of the valve circle and the angle COB 
is th? required angle of advance. We also know that a line drawn perpendic- 
ular to B O and tangent to the steam-lap circle cuts the valve travel circle 
at the points of admission and cut-off, respectively. Therefore, draw S T 2 
so it will be tangent to the steam-lap circle and perpendicular to B at H. 
The points S and T 2 are the points of head-end admission and cut-off, respec- 
tively. It is to be noted, also, that this line S T 2 is tangent to the lead circle, 
which fulfills another condition of the property of the Zeuner. 

To locate the other events for the head and crank ends, draw lines per- 
pendicular to B E and tangent to the steam- and exhaust-lap circles, and 
the points where these lines cut the valve travel circles will be the required 



VALVE GEARS 



25 



points. In the same manner, the several other points in the figure have been 

located. 

To find the per cent of stroke at which the several events occur, take a 

radius proportionately equal to the length of the connecting rod and describe 

R 
the arcs shown. As — is \, L equals 5 R. But R is one-half the valve travel, 



i.e., 1.5 inches. 



L = 5X1.5 
= 7.5 inches 



Now, with a radius of 7.5 inches and with a center on the horizontal 
line through the center of the valve travel circle produced to the left of the 



HEAD 



CUT-OFF HEADEND' 



COMPRESSION 
CRANK END 



RELEASE 
HEADEND 




RELEASE 
CRANK END 



COMPRESS/ON 
HEAD END 



Fig. 26. 



CUT-OFF CRANM END 



Diagram for Finding Events for Head and Crank Ends; Lead, Valve Travel, and 
Laps being Given 



vertical line C F, sweep the arcs shown from the points of admission, cut-off, 
etc., on the head and crank ends. Remembering that the head-end events 
are measured from the head-end dead center and the crank-end events 
from the crank-end dead center, measure the distance A T\. This distance, 
2.03 inches, divided by the valve travel 3 inches, and multiplied by 100, giveo 
the per cent cut-off on the head end, that is, 



2.03 



X 100=6, 



cut-off 
h.e. 



26 VALVE GEARS 

In like manner, -measure the distance D W for the crank-end cut-off, 
which we find is 1.8 inches. Then 

1.8 

XI 00 =60% cut-off 

3 h.e. 

Continuing this procedure for the other events, the final results obtained from 
the diagram will be 

Event Head End Crank End 

Admission 98 per cent 98 per cent 

Cut-off 67 per cent 60 per cent 

Release 93 per cent 91 per cent 

Compression 16 per cent 12| per cent 

Angle of advance 6 — 40 degrees 

Example 3. Given an engine having 30 per cent cut-off on the head 
end; maximum port opening of f inch; and lead on the head end tV inch. The 

laps are to be equal; compression on the head end is 25 per cent; and — equals 

L 
\. Construct the Zeuner diagram. 

Solution. In Fig. 27, lay off E F to represent the maximum port open- 
ing f inch; F G the lead ^ inch; and erect perpendiculars E J, G H, F I. On 
any point as O on the line G H, draw a trial circle such as A B C D, which in 
this case was assumed to be 1 inch in diameter. Since cut-off on the head end 
occurs at 30 per cent of the stroke, locate the direction of the crank position P 
for this position. This direction will hold for any valve travel. Draw M 
perpendicular to P, cutting F I at K. Bisect the angle F K M by K N. 
On K N &s & center line, find by trial a radius and center, such that a circle 
when described will pass through O and be tangent to E J. The center is 
found to be at L and the distance L is the radius of the required valve circle. 
With L as a center, draw a circle tangent to F I and K M. Such a circle will 
be the required steam-lap circle. To demonstrate why this construction is 
correct, it is only necessary to refer to the properties of the Zeuner diagram 
as given in connection with Fig. 24. Here it is shown that a line drawn per- 
pendicular to the crank position for the point of cut-off and tangent to the steam- 
lap circle cuts the valve travel circle at the extremity of the valve circle, as 
at B. Hence, O M fulfills this condition, which gives the extremity of the 
required valve travel circle at 0. In Fig. 24, it is also evident that the steam- 
lap circle is tangent to the perpendicular to the crank position for the given 
cut-off and is also tangent to a perpendicular to the horizontal center line 
drawn at the extremity of the maximum port opening. Therefore, this con- 
dition was fulfilled in establishing the required lap in Fig. 27. Having 
obtained the valve travel and lap, it remains to complete the diagram in 
order to determine the other conditions. In Fig. 28, the circles A B C D 
and abed are constructed on a diameter of 4rf inches and 4jt inches, 
respectively, the former being the value of the valve travel and the latter 
twice the steam lap, as fcund in Fig. 27. Locate the head-end cut-off 
at 30 per cent and draw the lead circle with a radius of tV inch. Locate 
the head-end compression of 25 per cent at I. Draw G H tangent to the 



VALVE GEARS 



27 



steam-lap and lead circles cutting the valve travel circle at G, thus estab- 
lishing the head-end admission. From the properties of the Zeuner diagram 
as discussed on pages 21 and 22, we know the diameter of the valve circles will 
be on a line bisecting the angle G H . Draw the line FOE bisecting this angle ; 
this line will be perpendicular to G H. Having established FOE and bear- 




Fig. 27. 



Diagram for Engine with Thirty Per Cent Cut-Off, Laps Equal, Compression 
Twenty-Five Per Cent 



ing in mind the demonstrations previously given, it is evident that a line drawn 
from the point of compression on the head end / perpendicular to F E will 
cut the valve travel circle at /, the point of head-end release. It is to be noted, 
however, that the line joining the points of release and compression on the 
head end lies on the same side of the center O as does the line joining the points 



28 



VALVE GEARS 



of admission and cut-off for the head end. This relation being opposite to that 
found in Fig. 24 means that instead of having exhaust lap with this valve, there, 
is inside clearance equal to N. With as a center and N as a radius, 
describe the clearance circle and complete the Zeuner by drawing the parallel 
lines K L and P Q, thus locating the remaining events of the stroke. In order 
to obtain the per cents of the events of the stroke, proceed as in Example 2. 
The results are tabulated as follows: 



Steam lap 

Inside clearance 

Valve travel 

Angle of advance 

Admission on both h.e. and c.e. 

Cut-off c. e. 

Compression c. e. 

Release c. e. 

Release h. e. 



= 2j2 inches 

= ts inch 

= 4 if inches 

= 62 degrees 

= 99 per cent (approx.) 

= 20 per cent 

= 18 per cent 

= 60 per cent 

= 72 per cent 



CUT-OFF HEAD END 
JO°/o 



RELEASE HEADEND 



COMPRESS/ON 
CRANK END 




C0MPRE3S101 
HEADEND ^5 <yo 



RELEASE CRANK END 



CUT-OFF CRANKEND 



Fig. 28. 



Diagram for Example 3 to Determine Admission, Compression, and 
Release at Crank and Head Ends 



The preceding problems involve nearly all of the properties 
of the Zeuner diagram and, if completely mastered by the student, 
should make the solution of other problems very much easier, 



VALVE GEARS 



29 



Effect of Changing Lap, Travel, or Angular Advance. We are 

now in a position to consider more in detail the effect of changing 




Fig. 29. Study of Effect of Changing Valve or its Setting 




Fig. 30. Study of Effect of Changing Angle of Advance 

in any way either the valve or the setting. Let us consider Fig. 
29, which is in every way like Fig. 23 except that all unnecessary 



30 



VALVE GEARS 



TABLE I 
Effect of Changing Lap, Travel, and Angular Advance 



Event 


Increasing 
Outside Lap 


Increasing 
Inside Lap 


Increasing 
Travel 


Increasing 
Angular 
Advance 


Admission 
Expansion 

Exhaust 
Compression 


\ Is later 

j Ceases sooner 

j Is earlier 

j Continues longer 

Unchanged 

J Begins at same point 
1 Continues longer 


Not changed 

j Beginning unchanged 
'/ Continues longer 

j Occurs later 
1 Ceases sooner 

j Begins sooner 
1 Continues longer 


J Begins earlier 
| Continues longer 

j Begins later 
'{ Ceases sooner 

| Begins earlier 
) Ceases later 

j Begins later 
j Ceases sooner 


J Begins earlier 
1 Same period 

j Begins earlier 
1 Same period 

( Begins earlier 
"l Same period 

j Begins earlier 
'( Same period 



letters and lines are omitted to avoid confusion. If the outside 
lap, or steam lap, is increased an amount equal to N M, the admis- 
sion will take place later, viz, at crank position A 2 ; the lead will 
be reduced to I G and cut-off will take place earlier, viz, at 0C 2 . 




Fig. 31. Study of Effect of Changing Eccentricity 

If the outside, or steam lap, is reduced a like amount, the contrary 
effects will be observed. If the inside lap, or exhaust lap, is increased 
an amount equal to L S, the release will take place later at the crank 
position B 2 > and compression will take place earlier at D 2 - The 



VALVE GEARS 31 

contrary effect will be observed by decreasing the inside lap, or 
exhaust lap. 

If the angular advance is increased, all the events will occur 
earlier, as is evident from Fig. 30. The crank revolves in the direc- 
tion indicated by the arrow and A 2 (new position of admission) is 
ahead of A\ the old position. 

If the eccentricity is increased, Fig. 31, the valve travel will 
increase and admission will take place earlier at A 2 ; the lead will 
be increased an amount equal to IJ 2 , and cut-off will take place 
later at C 2 . Release will be earlier at OB 2 and compression will 
be later at D 2 . The upper valve circle will now cut the arc drawn 
from as a center, with a radius equal to the outside lap plus the 
width of steam port, in the points W 2 and H 2 , and the admission 
port will be open wide while the crank is moving from W 2 to H 2 . 
Similarly, the lower valve circle cuts the arc drawn from as a 
center, with a radius equal to the inside lap plus the width of steam 
port, in the points W\ and Hi. The steam port is then wide open 
to exhaust while the crank is moving from W\ to H i. From the 
above, it will be seen that the periods are all changed by changing 
the travel, thus admission and exhaust begin sooner and last longer, 
and expansion and compression begin later and cease sooner. 

For convenience, these results are collected in Table I, which 
shows the effect of changing the laps, travel, and angular advance. 

There are, of course, all sorts of combinations that would make 
up different problems, but they can all be solved in the same general 
way, as they are modifications of the problems solved above, 

DESIGN OF SLIDE VALVE 

1j designing a slide valve, some of the variables are assumed 
and the others are found by means of the diagrams presented above. 
These diagrams show only the dimensions of the inside and outside 
laps and travel of valve; the other dimensions of the valve and seat 
must be calculated. 

Area of Steam Port. Steam Supply Pipe. It is generally con- 
ceded by authorities that the pipes supplying steam to steam engines 
should be of such dimensions that the mean velocity of steam in 
them would not exceed 6,000 feet per minute. If the velocity of 
steam exceeds 6,000 feet per minute, there will be a very appreciable 



32 VALVE GEARS 

loss of pressure, which is objectionable. In computing the size of 

a steam supply pipe for an engine, the assumption is made that the 

cylinder is filled at each stroke. The volume of steam passing 

through the steam pipe must equal the total volume of steam used 

by the cylinder. 

Let d equal diameter of steam pipe in inches; D equal diameter 

of cylinder in inches; L equal length of stroke in feet; and N equal 

revolutions per minute (r. p. m.). 

~d 2 

The area of the steam pipe in square feet would be and 

tZ)2 4 4X144 

that of the cvlinder would be The total volume of steam 

4X144 ,<r- 

flowing through the pipe per minute would be — : X 6000. Dis- 

4X144 

regarding the volume of the piston rod, the total volume of steam 

-D 2 

used bv the cvlinder in one minute would be X2LN. 

4X144 

Since the volume of steam flowing through the pipe per minute 

must equal that used by the cylinder in the same time, we can equate 

the two expressions; that is, 



Solving, 



4X144 


X6000 


4X144 


X2LN 




d 2 = 


D*LN 

3000 






d = 


dVln 





54.772 

Exhaust Pipe. For exhaust pipes, the mean velocity of steam 
is taken as 4,000 feet per minute. Therefore 



Tzd 2 -D 2 

X4000 = — —X2LN 



Solving, 



4X144 4X1' 

D 2 LN 



d 2 = 



2000 

dVln 

44.721 



VALVE GEARS . 33 

Example. Suppose an engine is 10 inches X 18 inches, and makes 180 
revolutions per minute. Determine the diameters of the steam and exhaust 
pipes. 

Solution. Substituting in the equation 

dVln 

d ~ 54.772 
gives for the diameter of the steam supply pipe 

10l/l.5 X 180 



d = 



54.772 
164.3 



54.772 
ss 3 inches 

The required diameter of exhaust pipe would be 

dVln 



d = 



44.721 



10l/l.5X180 

44.721 
164.3 



44.721 
= 3.67 inches 
A 4-inch pipe would probably be used. 

In practice different builders use different formulas, but all 
are derived from the fundamental assumptions made above, with 
certain constants added for different types of engines. The size 
of both steam and exhaust pipes required for engines of the same 
class is not affected in any marked degree by different types of 
valve gears. 

For a very large engine cutting off early, the allowable velocity 
may be taken as 8,000 feet per minute instead of 6,000 feet. 

Width of Steam Port. The port opening at admission should 
give nearly as great an area as the steam pipe in order to prevent 
loss of pressure due to wire-drawing, but the actual width of the 
port should be great enough for the free exhaust of steam. It is 
well to have the steam port a little larger than the area of the steam 
pipe, then with a port opening of six-tenths to nine-tenths of the 
port area for admission and full port opening at exhaust, satisfactory 
conditions will result. 



34 VALVE GEARS 

The length of the ports is usually made about eight-tenths the 
diameter of the cylinder. Then in the 10-inch X 18-inch engine, 
the steam ports would be .8X10, or 8 inches long. If the area for 
admitting steam is 7.0686 square inches (corresponding to a pipe 
3 inches in diameter) and the length of port is 8 inches, the width 

will be — , or .8836 inch — about J inch. 

8 

The width of port opening would be about .9 X. 8836, or .79524 
inch — about ff inch. 

Width of Exhaust Port. When the slid, valve is at its maxi- 
mum displacement, the valve overlapping the exhaust port, as shown 
in Fig. 7, reduces the area more or less. In designing the valve, 
the exhaust port should be of such a width that the maximum dis- 
placement of the valve does not reduce the area of the exhaust port 
to less than the area of the steam port. It is not advisable to make 
the exhaust port too large, for this increases the size of the valve 
and thus causes excessive friction. 

The height of the exhaust cavity should never be less than 
the width of the steam port and may be made much higher to 
advantage. 

Width of Bridge. The bridge must be of si fiicient width so 
that the outside edges of the valve can not uncov j - che exhaust port. 
The width of the steam port plus the width of tl .z outside lap plus 
the width of the bridge must be greater than the maximum displace- 
ment. 

The width of the bridges should not be iess than the thickness 
of the cylinder wall in order to make a good casting. 

Point of Cut=Off . In the study of Steam Engine Indicators, 
it was shown that if the point of cut-off is too early, the other events 
are not good. If a plain slide valve is used with an automatic cut- 
off, the point of cut-off is controlled either by changing the eccen- 
tricity or by changing the angular advance. Either of these meth- 
ods will accomplish the result at the expense of the compression, 
which at a very early cut-off may be excessive. Except for locomo- 
tives and high-speed engines, where compression is an advantage, 
the plain slide valve is not arranged to cut off earlier than one-half 
or two-thirds stroke. If an earlier cut-off is desired, large outside 
^ps are necessary. 



VALVE GEARS 35 

Lead. Tht lead of stationary engines varies from zero to f 
inch according to the style of engine and type of valve gear. An 
engine having high compression that compresses the steam nearly 
to boiler pressure will give good results with little or no lead. If the 
ports are small and the clearance large, there should be considerable 
lead in order to insure full initial pressure on the piston at the begin- 
ning of the stroke. Valves that open slowly require more lead than 
quick-acting valves. 

ILLUSTRATIVE PROBLEM 

Example. Design and lay out the valve and valve seat for an engine 
of cylinder diameter 10 inches, stroke 18 inches, revolutions 180 per minute, 
lead angle 3 degrees, cut-off equal at both ends and taking place at 75 per cent 
of stroke, maximum port opening .9 area of steam pipe, compression 15 per 
cent of the stroke at both ends, and length of connecting rod 3 feet. 

Solution. The piston displacement, or cylinder volume, will be 

3 1416 X 10 2 

— X 18 = 1413.7 cubic inches, or .818 cubic feet. 

4 

If the engine makes 180 revolutions, neglecting the volume of the piston 

rod, it will use 2 X 180 X. 818 =294.48 cubic feet of steam per minute. Steam 

294.48 

pipe area= = .0491 square feet, or 7.07 square inches. 

p F 6000 

This 7.07 square inches would also be the least possible area of the steam 

ports. If the length of port is made eight-tenths the diameter of cylinder, the 

width will be — : — = .88 inch, or about f inch. The width of maximum port 

8 

opening will be . 9 X. 88 = .792 inch, or nearly xt inch. 

Zeuner Diagram. It will be necessary to draw a separate valve circle 
for each end of the cylinder. First, consider the head end. The valve travel 
not being known, we shall lay off X Y on an assumption of 6 inches travel and 
draw the eccentric circle as shown in Fig. 32. Lay off the lead angle X A\ = 
3 degrees. Lay off X Ci = .75 of the assumed valve travel A\ inches. Draw the 
arc C1C2, as previously explained, and draw C\ which will be the crank posi- 
tion at the point of cut-off. The radius of the arc C1C2 will be equal to four 
times the radius of the eccentric circle, or 12 inches, because the connecting rod 
is four times the length of the crank. Let the line Ei bisect the angle A iO Ci, 
and on Ei draw the valve circle. Vi (=0 Ki) is then the outside lap, 
with these assumed conditions. Drawing the lap circle, the maximum port 
opening E1N1 is found to equal 1^6 inches, although yf is all that is neces- 
sary. The assumed eccentricity is 3 inches, therefore the probable eccen- 
tricity is found from the proportion 

x . o . . 16 . 1 16 

x = 1 xi inches 

Now draw a new eccentric circle with a radius of lxi inches and a new 
valve circle with a diameter E> lji inches. K* is now the outside lap and 



36 



VALVE GEARS 



the maximum port opening is equal to £2 Ns, which from actual measurement 
is found to be xl inch. The outside lap K 2 (=0 V2) is ff mcn an d the lead 
I J is -is inch. 

Produce EiO to F and draw another valve circle. We shall use this valve 
circle to determine the outside laps and lead for the crank end of the cylinder. 
Pince the cut-off is to be .75 of the stroke, we may lay off H 2 =0 Ci and, with 
a radius of 12 inches, draw the arc H1H2. Then, as already explained, Hi 



*ti 



Wj 



_£? 



& 



VT 



N* 



\K> 



JW 



10 



\*2 



Y 



"7 



<F 



Fig. 32. Zeuner Diagram for Design of Valve and Valve Seat in Problem Page 35 

will be the crank angle at cut-off on the return stroke. B, the outside lap, 
will be £f inch. Draw the lap circle intersecting the valve circle at D. Then 
DA 2 is the crank position at admission on the return stroke and L M,l inch 
is the lead on the crank end of the cylinder. The maximum port opening will 
always be greater at the crank end than at the head end because the crank end 
lap is less in order to get the equal cut-off. If the laps were equal, of course 
the port openings would be equal. 



VALVE GEARS 



37 



Now lay off X G 2 equal to fifteen-himdredths of X Y and find the crank 
position Gu This is the compression on the head end of the cylinder and 
gives an inside lap on this end of ^ inch, which is equal to P. Draw the lap 
circle P Q, which allows us to draw through Q the crank line R, which is 
the release on the forward stroke. 

Lay off Y S 2 (=X G 2 ) equal to fifteen-hundredths of X F, and construct 
the crank line Si, which is the crank position at the crank -end compression. 
O Si intersects the valve circle at T, giving T, ■& inch, as the inside lap on 
the crank end. Draw this lap circle, which will intersect the valve circle at U. 
This enables us to draw U W, the crank position at release, on the return 
stroke. 

Layout of Valve. From the data determined by means of these diagrams, 
the valve may now be laid out. For convenience let us tabulate the results 
obtained as follows: 



Data 


Head End 


Crank End 


Cut-off (per cent 








of stroke) 


75 per cent 


75 per cent 


Outside lap 


fi inch 




§f inch 


Inside lap 


■£2 inch 




iV inch 


Lead 


W2 inch 




f inch 


Port opening 


if inch 




ltV inches 


Width of port 


1 inch 




| inch 


Fig. 33 shows this valve 


in section. 


Let 


us begin at the end having the 



largest inside lap or, in this case, at the crank end. Lay out the steam port f 
inch wide and the crank-end outside lap if inch. The bridge will be, say, | 
inch wide. From the inner edge of the steam port, lay off the crank-end inside 
lap tV inch. When the valve moves to the left, the point E 2 will travel 1 xe 
inches — a distance equal to the eccentricity — and in this position of extreme 
displacement, the exhaust port EiF must be open an amount at least equal 
to the steam port, f inch. 
Therefore, we lay off Ei F equal 
to lxi inches + f inch = 2f\ 
inches. The inside lap over- 
laps the bridge Dearly § inch, 
so that we shall have to make 
the exhaust port opening equal 
to 2f inches. Lay off f inch 
again for the bridge and meas- 
ure back ^2 inch, equal to the 
head-end inside lap. The port 
is I inch wide, and the head-end outside lap of §f inch completes the outline 
of the valve seat. 

Reversing Simple Engine. In the operation of a simple engine 

having a plain slide valve or a piston valve, it sometimes becomes 

necessary to reverse the direction of rotation of the engine shaft. 

Remembering the principles presented in the foregoing study of the 

Zeuner diagram, this is not a difficult task. 




Fig. 33. 



Section of Valve Designed from 
Diagram Fig: 32 



38 



VALVE GEARS 



It is proposed to here show fast, how an engine may be reversed 
with a direct valve, engine running over; second, with a direct valve, 
engine running under; third, with an indirect valve, engine running 
over; and fourth, with an indirect valve, engine running under. 





Fig. 34. Section Showing Lead of Valve, 
Engine Running Over 



Fig. 35. Diagram for Direct Valve, 
Engine Running Over 



Definitions. Before explaining the operation for obtaining 
the above, it is well to have an understanding of the meaning of the 
terms "direct" and "indirect" as applied to a valve, and of "run- 
ning over" and "under" as applied to an engine. 

A valve is said to be a direct, or outside admission, valve, wnen 
at the beginning of the stroke the valve and the piston are moving 
in the same direction, as indicated by the arrows in Fig. 6. It is 





Fig. 36. Section Showing Lead for Direct 
Valve, Engine Running Under 



Fig. 37. Diagram for Direct Valve, 
Engine Running Under 



also to be noted that steam is being admitted to the cylinder by the 
outer edge of the valve, which is the reason for calling it an outside 
admission valve. 

If, in Fig. 6, the valve should be moving in the opposite direc- 
tion from that shown and steam should be entering the cylinder by 
the inner edge of the valve, the valve would then be said to be an 
indirect, or inside admission, valve. 



VALVE GEARS 39 

Most plain slide valves are of the outside admission type, while 
most piston valves are of the inside admission type. 

An engine is said to be running over, if, when the piston is mov- 
ing from the head end toward the crank end, the moving parts, such 
as connecting rod, crank, etc., are above the center line, as shown in 
Fig. 17. The engine is said to be running under when the above 
mentioned parts are below the center line when the piston is mov- 
ing from the head end toward the crank end. 

Direct Valve, Engine Running Over. In Fig. 34, let the valve 
have lead equal to that shown. Since this is a direct valve, engine 
running over, the valve will be to the right of its mid-position and 
moving to the right, hence the eccentric will be (90+#) degrees ahead 
of the crank. If the engine is on the head-end dead center, the eccen- 
tric would be at E, that is, (90+0) degrees ahead of the crank. The 
right and left valve circles will be located in the second and fourth 
quadrants, respectively, as shown in Fig. 35. 

Direct Valve, Engine Running Under. With a direct valve, 
engine having lead and running under, as illustrated in Figs. 36 
and 37, the valve will be in the same relative position as in the former 
case, when the crank is on the head-end dead center. In this posi- 
tion the valve must be to the right of its mid-position and moving 
towards the right, hence the eccentric must be, as shown at E, Fig. 
37, an angular distance of (90+#) degrees ahead of the crank. 

The right and left valve circles will be located in the first and 
third quadrants, respectively, as shown in Fig. 37. 

It is to be noted on comparing the position of the eccentric in 
Figs. 35 and 37 that both of the eccentric positions make an angle 
equal to the angle of advance with the vertical. Therefore, to 
reverse a direct valve, engine running over, turn the eccentric around 
the shaft, in the direction in which the engine is running, by an angle 
of (180 — 20) degrees, or turn the eccentric ahead of the crank, in 
the direction in which the engine is to run, an angle of (90+0) degrees. 

Indirect Valve, Engine Running Over. An indirect valve engine 
running over is illustrated in Figs. 38 and 39. Remembering that 
the valve must be moving to the left as the piston moves from the 
head end toward the crank end, and that the valve must be dis- 
placed by an amount equal to the lap plus the lead to the left of its 
mid-position, the eccentric must be below the horizontal and behind 



40 



VALVE GEARS 



the crank an angular distance of (90 — 0) degrees. Hence, it is 
located at E, Fig. 39. The right and left valve circles will be located 
in the fourth and second quadrants, respectively 





Fig. 38. Section of Indirect Valve, 
Engine Running Over 



Fig. 39. Diagram of Indirect Valve, 
Engine Running Over 



Indirect Valve, Engine Running Under. To locate the eccen- 
tric for an indirect valve engine having lead and running under (see 
Figs. 40 and 41), proceed as before. The eccentric will be found at 
E, Fig. 41, and the right and left valve circles will be located in the 
first and third quadrants, respectively. 

An examination of Figs. 38 to 41 will disclose the fact that to 
reverse an engine using an indirect valve, it is only necessary to 
turn the eccentric through an angle of (180 — 2/9) degrees in the direc- 
tion in which the engine shaft is turning or, in other words, the pro- 
cedure is the same as for a direct valve. 





Fig. 40. Section of Indirect Valve, 
JEngine Running Under 



Fig. 41. Diagram for Indirect Valve, 
Engine Running Under 



Comparisons and Comments. A comparison of Figs. 34 and 35 
with 38 and 39 will indicate the relative positions of the eccentric 
for an engine running over with a direct valve and for one running 
over with an indirect valve. It is evident that in the first case, the 
eccentric precedes the crank by an angle of (90 -f 0) degrees, whereas 



VALVE GEARS 41 

in the second, the eccentric follows the crank by an angle of (90 — 6) 
degrees. This same condition is true for two engines running under, 
one using a direct valve and the other an indirect. 

As an aid in locating the valve travel circles after the eccentric 
position has been determined, remember that the quadrant separated 
by a vertical line through the center, from the quadrant contain- 
ing the eccentric position, is the quadrant in which the right valve 
travel circle is to be located. 

All of the study on the Zeuner valve diagram thus far has to 
do with an engine running over having a direct valve. After the 
location of the eccentric position has been determined for the above 
various conditions, the construction of the Zeuner diagram should 
be a simple matter. 

The principles underlying the location of the eccentric for an 
engine running over or under and having a direct or indirect valve 
should be borne in mind when setting valves. 

VALVE SETTING 

Possible Adjustments. The principles of valve diagrams are 
useful in setting valves as well as in designing them. The valve is 
usually set as accurately as possible, and then, after indicator cards 
have been taken, the final adjustment can be made to correct slight 
irregularities. 

The slide valve is so designed that the laps can not be altered 
without considerable labor, and the throw or eccentricity of the 
eccentric, which determines the travel of the valve, is usually fixed. 
The adjustable parts are commonly the length of the valve spindle 
and the angular advance of the eccentric. 

By lengthening or shortening the valve spindle, the valve is 
made to travel an equal distance each side of the mid-position. Mov- 
ing the eccentric on the shaft makes the action of the valve earlier 
or later as the angular advance is increased or decreased. 

To Put Engine on Center. It is usual to put the engine on 
center before setting the valve. First, put the engine in a position 
where the piston has nearly completed the outward stroke and 
make a mark M 1} Fig. 42, on the guide opposite the corner of the 
crosshead at some convenient place. Also make a mark P with 
a center punch on the frame of the engine near the crank disk. With 



42 



VALVE GEARS 



this mark P as a center, describe an arc C on the wheel rim with a 
tram. * 

Turn the engine past the center until the mark on the guide 
again corresponds with the corner of the crosshead and make another 




Fig. 42. Sketch of Engine, Showing Method of Putting Engine on Center 

mark D on the wheel with the tram, keeping the same center. With 
the center of the pulley, or crank disk, as a center, describe an arc 
CD on the rim, which intersects the two ares drawn with the tram. 
Bisect the arc CD and turn the engine until the new point is dis- 
tant from the point P an amount equal to the length of the tram, 
in which position the engine will be on center. 

The engine should always be moved in the direction in which 
it is to run so that the lost motion of the wrist pin and crank pin 




Fig. 43. Diagram Showing How Valve is Set for Equal Lead 

will be taken up the right way. In case the engine has been moved 
too far at any time, it should be turned back beyond the desired point 
while the engine is moving in the proper direction. In this manner, 
the dead center can be located for both the head and crank ends. 



*A tram is a steel rod with its ends bent at right angles and sharpened. 



VALVE GEARS 43 

To Set Valve for Equal Lead. After locating the dead center 
points as described above the next step is to locate what are known 
as the port marks. In Fig. 43 move the valve to the left until cut- 
off occurs on the head end or until the edge of the valve at B is at M 2 . 
Then, with a center C on some fixed point on the cylinder or engine 
frame, describe with a tram the arc F G on the valve rod. Continue 
the rotation of the engine in the same direction until cut-off takes 
place at the crank end. Then with the same tram and center C, 
sweep the arc D E on the valve rod. Draw the center line H I and 
where this center line cuts the arcs F G and D E, mark the points 
J and K, respectively, which points are known as the port marks. 
Bisect the distance between J and K, thus establishing the point 0. 
When one tram point is in C and the other just enters the point J, 
the valve is just cutting off on the head end; and when the tram 
point coincides with C and K, it is an indication that cut-off is occur- 
ring on the crank end, hence a basis of comparison has been estab- 
lished for the two ends. Place the engine on the forward dead cen- 
ter and sweep the arc LiM±. The distance between the arcs LiMi 
and F G, which is equal to JQ 1} represents the amount the valve 
extends over the port when the engine is on the head-end dead cen- 
ter. In a like manner, establish the arc NiPi when the engine is 
on the crank-end dead center, in which position the valve overlaps 
the steam port the distance KQ 2 . In order to have equal travel 
of the valve on either side of its mid-position, the distance JQi 
should equal K Q 2 . If necessary to equalize these distances, lengthen 
or shorten the valve stem as required. Having secured an equal 
valve travel, place the engine on the forward dead center. Since 
the engine is running under (see Fig. 37), the eccentric will be found 
(90 + 0) degrees ahead of the crank in the direction the engine 
is to run. Lay off on the valve stem the distances J Q 3 and KQ A 
equal to the required lead. With the tram pomt in C and 
the engine placed on the head-end dead center, turn the eccen- 
tric in the direction in which the engine is to run — which is wider 
in this case — until the arc R S passes through the point Q s . Fasten 
the eccentric and turn the engine around until it is on the crank- 
end dead center. Sweep another arc as T U with the tram. If 
this arc passes through the point Q 4 , then the valve is correctly set 
for equal lead, that is, JQ d is equal to KQ±. 1^ however, the arc T V 



44 VALVE GEARS 

does not pass through the required point Q 4 , but falls beyond, it is 
an indication of unequal lead, so a correction must be made. Sup- 
pose, for instance, that when the crank was placed on the crank-end 
dead center, the arc described from C fell at X Y instead of T U, 
then it is obvious that the crank end has more lead than the head 
end. To make a correction for this inequality, find the difference 
between the lead on the head and crank ends — which in this case is 
equal to the distance Q±Q b — and correct half of the difference on the 
valve stem and the other half by altering the angle of advance. In 

this case, the valve stem should be lengthened by the amount ^-^, 

which would increase the lead on the head end by that amount 
and decrease it by the same amount on the crank end. After 
establishing an equal travel of the valve by adjusting the length 
of the valve stem, thus giving an equal amount of lead at each end, 
the desired amount of lead may be obtained by changing the angle 
of advance. To obtain the required lead in this case, it would be 
necessary to reduce the angle of advance. It may be necessary to 
make several trials before the desired results are obtained, this 
being particularly true if working on an engine having lost motion 
in the various parts. In order to eliminate the effect of lost motion 
in so far as possible, the engine should always be turned in the direc- 
tion which it is to run. 

In case it is difficult to turn an engine, the following method 
may be used. First, loosen the eccentric on the shaft and turn it 
around until it gives a maximum port opening first at one end and 
then at the other. If the maximum port openings are not equal, 
make them so by changing the length of the valve spindle by half 
the difference. When the above adjustment has been made, set 
the engine on dead center and give the valve the proper lead by turn- 
ing the eccentric on the shaft. The angular advance is thus adjusted. 

To Set Valve for Equal Cut=Off. To set the valve for equal 
cut-off, observe the preliminary steps of locating on the valve stem 
the dead-center points, port marks, and equal travel of the valve to 
either side of its mid-position, as described in connection with set- 
ting the valves for equal lead. 

Assume that it is desired to set the valves for an equal cut-off 
of 75 per cent. On the guides of the engine illustrated in Fig. 42, 



VALVE GEARS 45 

locate the points M 2 and M s , corresponding to the extreme positions 
of the edge of the crosshead, or a given point on the crosshead. The 
distance M 2 M S represents the stroke of the piston, so when 75 per 
cent cut-off occurs, the reference point on the crosshead should be 
at a point J, which is 75 per cent of the stroke M 2 M 3 for the crank 
end and at the point L for 75 per cent cut-off on the head end. Re- 
membering that the points J and K on the valve stem in Fig. 43 
represent points of cut-off, all required reference points needed are 
known. Turn the engine over in the direction indicated in Fig. 
42 until the reference point on the crosshead corresponds to the refer- 
ence point on the guide, as L, for the head-end cut-off. Then with 
the tram in the center C, Fig. 43, describe an arc, say, LiMi. Con- 
tinue the rotation of the engine in the same direction until the piston 
has completed the forward stroke and has returned to the point 
where the reference lines on the crosshead and the guide / coincide. 
Tram the valve stem as before, locating the arc, say, iViPi. Since 
the tram should coincide with the arcs F G and D E for the 
head-end and crank-end cut-off, respectively, it is therefore evident 
that with the tram coinciding with LiM i and iViPi that the required 
cut-off is not obtained but occurs too early. Since the distances 
QiJ and Q 2 K are equal, the length of the valve stem does not need 
to be disturbed. To make cut-off occur later, decrease the angle of 
advance by moving the eccentric opposite to the direction in which 
the engine is to run. For instance, with the engine standing so 
that the point L, Fig. 42, and the end of the crosshead are 
coincident, move the eccentric until the tram points coincide with 
C and J, Fig. 43. Try the points for the cut-off on the crank 
end, and if the tram fits easily into C and K, then the valve is 
set correctly. If, however, the tram points do not fit into the 
points C and K, continue the operation until the desired results are 
obtained. 

From the above discussion, two points have been estab- 
lished : 

(1) Moving the valve on the valve rod changes the corresponding 
events the same on both ends, one being made earlier and 
the other later. That is, if the cut-off is made earlier on the 
head end, it will be later on the crank end, and so on for the 
other events. 



46 



VALVE GEARS . 



(2) Gloving the eccentric on the shaft or changing the angle of 
advance changes the corresponding events the same for both- 
ends, both being made earlier or both later. 

MODIFICATIONS OF THE SLIDE VALVE 

Balancing Steam Pressure. The ordinary slide valve is most 
suitable for small engines. For engines of large size, some method 
must be employed to balance the steam pressure on the back of the 
valve. With large valves, such for instance as those of locomotives 
or large marine engines, a great force is exerted by the steam, and 
the valve is forced against its seat so hard that a large amount of 




Fig. 44. Section of Piston Valve and High-Pressure Cylinder of U. S. S. 
"Massachusetts" Showing Method of Balancing 

power is necessary to move it. This excessive pressure causes the 
valve to wear badly and is a dead loss to the engine. The larger 
the valve, the greater this loss will be. 

Piston Valve. To prevent excessive pressure on the back of 
the valve, the piston valve is commonly used, especially in marine 
engines. This valve consists of two pistons which cover and uncover 
the ports in precisely the same manner as the laps of the plain slide 
valve. These pistons are secured to the valve stem in an approved 
manner and are fitted with oacking rings. 



VALVE GEARS 



47 



The valve seat consists of two short cylinders or tubes accur- 
ately bored to fit the pistons of the valve. The port openings are 
not continuous as in the plain slide valve, but consist of many small 
openings, the bars of metal between these openings preventing the 
packing rings from springing out into the ports. 

Steam may be admitted to the middle of the steam chest and 
exhausted from the ends or vice versa. With the former method, 
the live steam is well separated from the exhaust, and the valve- 
rod stuffing box is exposed to exhaust steam only. This is a good 
arrangement for the high-pressure cylinder; if used for a cylinder 
in which there is a vacuum, air may leak into the exhaust space 
through the valve-rod stuffing box. With this arrangement, the 
steam laps must be inside and the exhaust laps on the outside ends. 

The piston valve may be laid out and designed by means of the 
Zeuner diagram just as if it were a plain slide valve, and the action 




Fig. 45. Section of Double-Ported Slide Valve 



is the same except that it is balanced so far as the steam pressure is 
concerned, the power to drive it being only that necessary to over- 
come the friction due to the spring rings. 

Fig. 44 shows a section of the piston valve ana the high-pressure 
cylinder for one of the engines of the U. S. S. "Massachusetts." 
This valve consists of two pistons connected by a sleeve through 
which the valve rod passes. This valve rod is prolonged to a small 
balancing piston, placed directly over the main valve. The upper 
end of the balancing cylinder does not admit steam, so that the 
steam pressure below the balancing piston will practically carry the 
weight of the piston valve, thus relieving the valve gear and making 
the balance more nearly complete. 

Double-Ported Valve. Sometimes it is impossible to get suffi- 
cient port opening for engines of large diameter and short stroke, 



48 



VALVE GEARS 



r^o 




Fig. 46. Trick Valve Shown in Mid-Position 



especially those having a plain slide valve with short travel. This 
difficulty may be overcome by means of the double-ported valve 
shown in Fig. 45. It is equivalent to two plain slide valves, each 
having its laps. The inner valve is similar to a plain slide valve 

except there is communica- 
tion between its exhaust space 
and the exhaust space of the 
outer valve. Each passage to 
the cylinder has two ports; a 
bridge separates the exhaust 
of the outer valve from the 
steam space of the inner valve, 
and the outer valve is made long enough to admit steam to the 
inner valve. 

This valve may be considered as equivalent to two equal slide 
valves of the same travel, each having one-half the total port open- 
ing. To admit the same amount of steam as a plain slide valve, 
the double-ported valve requires but half the valve travel; this is 
advantageous in high-speed engines. 

To balance the excessive steam pressure, the back of the valve 
is sometimes provided with a projecting ring which is fitted to a 
similar ring within the top of the valve chest. These rings are 
planed true and fit so that steam is prevented from acting on the 
back of the valve. 

Trick Valve. The defect of the plain slide valve, due to the 
slowness in opening and closing, is largely remedied in the trick valve, 




Fig. 47. Trick Valve Showing Admission 
of Steam Just Beginning 



Fig. 48. Trick Valve at Extreme Right 
Position with Steam Port Open Wide 



which is so made that a double volume of steam enters during admis- 
sion. Thus a quick and full opening of the port is obtained with a 
small valve travel. 



VALVE GEARS 



49 



In Fig. 46 the valve is shown in mid-position. It is similar to 
a plain slide valve except that there is a passage P P through it. 
It has an outside lap and an inside lap 7. The seat is raised and 
has steam ports S S, bridges B B, and exhaust port E. If the valve 
moves to the right a distance equal to the outside lap plus the lead, 
it will be in the position shown in Fig. 47. Steam will be admitted 
at the extreme left edge of the valve just the same as though it were 
a plain slide valve; also, since steam surrounds the valve, it will be 
admitted through the passage as shown in Fig. 47. If the lead is 
the same as for a plain slide valve, re inch for instance, this valve 
would give double the port opening, that is, f inch, when the valve 
was open a distance equal to the lead. 




Fig. 49. 



Obtaining Perfect Balance by Use of Double-Ported Piston Valve and Double- 
Ported Slide Valve in Compound Engine 



Fig. 48 shows the valve when it is in extreme position to the 
right and the port is full open to steam. 

Piston valves are also made with a passage similar to that of 
the trick valve for double admission, that used with the Armington 
and Sims engine being, perhaps, the best example. 

Application of Various Types. Piston valves are commonly 
used on the high and intermediate cylinders of triple-expansion 
engines, and if well made and fitted with spring rings, should not 
leak. Small piston valves are often made without packing rings; 
but even if they fit accurately when new, they soon become worn 
and cause trouble. 



50 VALVE GEARS 

The double-ported valve, the trick valve, and others, often have 
some device for relieving the pressure, such as a bronze ring or 
cylinder fastened to the back of the valve. This ring is pressed by 
springs against a finished surface of the valve chest cover, and the' 
space thus enclosed by the ring may be connected to the exhaust. 
There are numerous devices for balancing valves, but they are 
usually more or less expensive and are liable to cause trouble from 
leakage. 

Fig. 49 well illustrates the application of a double-ported piston 
valve and a double-ported slide valve to a compound engine. It 
also shows a method used for obtaining a perfect balance. The 
piston valve on this engine is a hollow inside admission valve. The 
steam passes from the cavity A through the double ports in the piston 
valve, forcing the high pressure piston to the right, which action 
causes the exhaust steam to pass out of the high pressure cylinder 
through the passage B into the steam chest of the low pressure cylin- 
der. The steam passes around the flat valve at C C into the low 
pressure cylinder. The steam back of the low pressure piston ^passes 
through the port D into the exhaust cavity. The pressure plate E 
is held against the flat slide valve by the springs H and 7, there 
being steam all around the pressure plate, as at F and G. The valve 
fits closely between the valve seat and pressure plate, but the pres- 
sure plate being supported at the sides eliminates the pressure between 
the valve and its seat. Both of these valves are said by the builders 
to be in perfect balance. 

Reversing Mechanism. In the early development of valve 
gears, it became necessary to devise some means of reversing the 
engine, hence it is found that a great many of the most prominent 
gears, such as the Stephenson, Walschaert, Marshall, and many 
others of more or less merit, embody the reversing feature. 

Reversing by Means of One Eccentric. At first, the reversing of 
an engine was accomplished by the use of one eccentric, there being 
two methods by which this was done. 

(1) The device shown in Fig. 50 was used on some of the earliest 
locomotives and marine engines, and may now be found as the 
reversing medium for engines used on small launches. The eccen- 
tric E is loose on the shaft between a fixed collar G and a hand wheel 
H. A stud projecting from the eccentric passes through a curved 



VALVE GEARS 



51 



slot in the disk of the wheel and can be clamped by a hand nut F. 
When running forward with the crank at C, the center of the eccen- 
tric is at A and the nut is clamped at F. To reverse, steam is shut 
off and, when the engine stops, the nut F is loosened and then moved 
to B and clamped; or, after F is loosened, the wheel, shaft, crank, 
and propeller are turned over by hand until B strikes the stud at F, 
where it is clamped. The engine will then run astern. 

(2) The eccentric was mounted on a sleeve, which could be 
moved longitudinally along the shaft of the engine by means of a 
lever. The sleeve had a spiral slot cut on the inside of it, which 
subtended an angle of (180—2 0) de- 
grees. This slot fitted over a radial 
pin on the shaft, so when the sleeve 
was pushed in or out by the lever, 
both the sleeve and the eccentric 
were turned through (180—2 0) de- 
grees, thus reversing the engine. 

Reversing by Means of Two Eccen- 
trics and Gab- Hooks. It is obvious 
from the foregoing that the method 
of reversing by shifting one eccentric 
is awkward and not well adapted to 
high speeds and large engines. It was 
a natural transition, therefore, from 
the one eccentric to the more con- 
venient reversing gears having two 
eccentrics, one set (90+0) degrees ahead of the crank for the for- 
ward motion and one (90+0) degrees behind the crank for the back- 
ward motion. At first, this arrangement was rather crude and 
objectionable in some respects, as will be noted later. The essential 
feature to be borne in mind with reference to a two-eccentric gear is, 
that the object is to have only one eccentric at a time operating the 
valve. In the early development, this was accomplished by using 
gabs or gab-hooks, which could be brought in contact with the valve 
rod at the pleasure of the operator. For instance, if the engineer 
wished to go forward, he would lower the arm R, Fig. 51, thus bring- 
ing Bi in contact with the valve rod at V. The valve would then 
be operated by the forward eccentric E\ and the engine would run 




Fig. 50. Early Reversing Device by- 
Means of One Eccentric 



52 



VALVE GEARS 



in the direction indicated by the arrow at E\. To reverse the engine, 
Bi would be disengaged and B 2 placed in connection with V. The 
valve would then be operated by the eccentric E 2 , and the engine 
would run in the direction indicated by the arrow at E 2 , which is the 
reverse of that indicated at E\. It is to be particularly noted that 
only one eccentric actuates the valve at one time. All reversing 
gears of the two-eccentric type carry out this principle to a greater 
or less extent. 

It will be obvious that the gab-hooks are an improvement over 
the shifting eccentric, but even they have certain objectionable fea- 
tures, the three principal ones being (1) the engine must have a slow 
speed of rotation; (2) the engine must be of such construction that 





Jp)~« ' 


f) 


3 £. _ 


6 


JVn 


P °A 


h(cy y 








*^£—- 


1/? G 



Fig. 51. Reversing Device Using Two Eccentrics and Gab-Hooks 



the reversal can be accomplished in a leisurely manner — it is not con- 
venient to reverse at a high speed with a gab-hook, but the engine 
must be turning slowly when the hook is dropped upon the pin; (3) 
the engine must be of such a type that it can be started by hand- 
working of the valves. 

Reversing by Means of Two Eccentrics and Curved or Straight 
Links. To overcome these objectionable features, a step forward 
was taken when the gab-hooks were replaced by the curved or straight 
link, which is now used in connection with almost all reversing gears. 
This was a decided improvement as it not only accomplished the 
reversing of the engine but also made possible a variation in the 
adjustment of the valve mechanism, which permitted much more 
economical distribution of steam in the cylinder. There are two 
general classes of valve gears that use the curved link and its neces- 



VALVE GEARS 



53 



sary attachments, namely, the shifting link or the[ stationary link 
type, and the radial gear type. 

The Stephenson gear is a worthy exponent of the shifting link 
type. The Walschaert, Joy, Marshall, and others are representatives 
of the radial gear type. 



SHIFTING LINK TYPE OF VALVE GEAR 

Stephenson Link Motion. As the Stephenson gear is one of 
the oldest reversing gears used and is perhaps the best known, a dis- 
cussion of its principal fea- 
tures is presented first. This 
gear has been successfully 
used on stationary, traction, 
and marine engines, but its 
largest and, perhaps, most 
successful application has been 
on American locomotives. 
This gear is illustrated in Fig. 
52. The two eccentrics Ei 
and E 2 , whose centers are at 
Ci and C 2 , respectively, are 
shown in their relative posi- 
tions when the crank OA is 
at the crank-end dead center. 
The eccentric rods Ri and R 2 
are connected by forked ends 
to the link pins H and G. The 
link consists of two curved 
bars bolted together in such a 
manner that they may slide 
by the link block N. On the 
link are three sets of trun- 
nions; the two outer ones, or 




Fig. 52. 



Stephenson Link Motion 



link pins, are fitted into the forked end of the eccentric rods, and 
the middle one. known as the saddle pin, is fitted into the end of 
the drag links F M. 

The valve stem has, at its lower end, a pivoted block A T , called 
the link block, provided with slotted sides through which the links 



54 



VALVE GEARS 



can slide. The reverse shaft, or rock shaft, K, here shown in the 
full gear * 'forward," may be turned until F moves to B; in this 
position the link will be pushed across the link block, and the valve 
will get its motion from the rod R 2 instead of from R lt as before. 
The link in this position would be in full gear "backward." 

From the foregoing, it is obvious that the Stephenson gear may 
be divided into three distinct mechanisms, each of which perform a 
definite function. First, the link motion proper, comprising the 
parts from the axle to the link; second, the adjusting gear, which is 
composed of the lifting shaft and reversing lever by which the power 
of the engine is controlled by lowering or raising the link; and, 
third, the valve and its attachments. The link motion proper is, 




Fig. 53. "Open Rod" Arrangement of Eccentric Rods in Stephenson Gear 

perhaps, the most important of the three, at least for the present 
study. Remembering that the link supplanted the gab-hook, it 
should be obvious that the eccentric rods and their connection to 
the link form a combination similar in action to the gab-hooks and 
valve rod, with some intervening parts which do not materially 
affect or change the operation. 

Relative Position of Eccentric Rods. In order to have clearly 
in mind just what action does take place when the link is shifted 
from one position to another, it is essential that the relative posi- 
tion of the eccentric rods be understood. They are designated as 
"open rods" when arranged as shown in Fig. 53, with the eccentric 
centers C and D on the same side of the axle as the link, and "crossed 
rods" when the rods cross as illustrated in Fig. 54. The location, 
length, and attachment of the eccentric rods to the link have a mate- 



VALVE GEARS 55 

rial effect upon the movement of the valve. Experience and calcu- 
lations have shown that the eccentric rods should not be shorter 
than eight times the throw of the eccentric. They are usually 
much longer than this. The distance between the eccentric rod 
pins should not be less than two and one-half times the throw of the 
eccentric. If the distance is less than this amount, the angle between 
the link and the block will be such that there will be an excessive 
slip of the block and undue stresses in the mechanism will be induced. 
The angularity of the eccentric rods produces irregularities in the 
movement of the valve, which can be largely compensated for b;y 
locating the saddle pin inside the center line of the arc, but not too 
far inside for then it would give a long slip of the link and be objec- 
tionable. The adjustment of the link also requires that special a^ + eo- 




Fig. 54. "Crossed Rod" Arrangement of Eccentric Rods in Stephenson Gear 

tion be given to the amount of lead at full gear as well as to the 
increase of lead produced by "hooking up" the engine. With "open 
rods" it will be seen that when in full gear the link block is at Gi, 
and that if, without turning the crank, the link is shifted to mid 
gear, then the link block moves to J, Fig. 53, and the valve must 
consequently be moved toward the right an amount equal to G\ J, 
thereby increasing the lead on the crank end of the cylinder. With 
"crossed rods," moving the link from full to mid gear moves the link 
block from G\ to J, Fig. 54, thus reducing the lead. It follows then 
that open rods give increasing lead from full toward mid gear, and 
that crossed rods give decreasing lead. With crossed rods there 
will be no lead when in mid gear. It will be apparent that the shorter 
the rods, the greater this increase or decrease will be. 



56 



VALVE GEARS 



The open rods are more generally used than the crossed rods; 
especially is this true in locomotive service. The feature of increas- 
ing lead from full to mid gear is the distinguishing characteristic of 
the Stephenson gear. When the engine is in full gear, so that the 
forward link pin Gi is on the center line as in Fig. 53, then only the 
eccentric C controls the valve, and the travel of the valve will be 
equal to twice the throw of the eccentric C. In other words, when 
in full gear, only one eccentric moves the valve, as was the case when 
using gab-hooks. As the link is raised, both of the eccentrics have 
an effect on the motion of the valve, the result being very much the 
same as if another controlling eccentric of shorter throw were intro- 
duced. The throw of this resultant eccentric would decrease until 
the center was reached, when it would be a minimum. Finally, the 



+ -- 




Fig. 55. Diagram of Stephenson Gear Showing Link Block and Rocker 



center of this resultant eccentric would be on the center line of the 
motion, midway between the two actual eccentrics. At this point, 
the radius of the resultant eccentric would be equal to the sum of 
the lap and lead in full gear. Therefore, in mid gear, the valve travel 
is equal to twice the sum of the lap and lead in mid gear. 

Location of Link Block. Nearly all marine engines, and some 
English locomotives, have their link blocks carried directly on the 
valve rod. American locomotives commonly use a rocker, one end 
of which carries the link block while the other moves the valve 
rod. This arrangement, indicated in Fig. 55, makes it possible to 
place the valve and steam chest above the cylinder. The position 
of the crank for the same valve position is just opposite that shown 
in Fig. 53, because the rocker reverses the direction of motion of 



VALVE GEARS 57 

the valve. While apparently the crossed rod arrangement is used, 
yet it is really the open rod arrangement and gives increasing lead 
toward mid gear. A rod from the bell crank lever on the reverse 
shaft E leads back to the engineer's cab and connects with the 
reverse lever. This lever moves over a notched arc and may be held 
by a latch in any one of the notches, thus setting the link in any 
position from mid gear to full gear, either forward or back. 

The Stephenson link is designed to give equal lead at both ends 
of the cylinder; but to accomplish this, the radius of the link arc 
(that is, an imaginary line in the center of the slot) must be equal to 
the distance from the center of this slot to the center of the eccentric. 
In Fig. 52, the radius of the link arc is equal to CiH and C 2 G. 

Exact equality of lead is not essential, and the radius of the 
link arc is sometimes made greater or less than stated above in order 
to aid in equalizing the cut-off; but the change should never be great 
enough to affect the leads. 

Application to Expansion and Cut-Off. Stephenson originally 
intended to use the link simply as a reversing gear, but soon found, 
however, that at intermediate points between the two positions of 
full gear, it would serve very w T ell as a means of varying the expan- 
sion and cut-off. Very soon, the link came to be used not only on 
locomotives and marine engines, but on stationary engines as well, 
in connection with the reverse shaft which was under the control of 
the governor. The mechanism proved to be too heavy to be easily 
moved by a governor and it has gradually fallen into disuse on sta- 
tionary engines excepting as a means of reversing. 

In marine practice, the variable expansion feature is of little 
value, for marine engines run under a steady load and the link is 
set either at full gear or at some fixed cut-off. For locomotives, 
however, the variable expansion is nearly as important as revers- 
ing. Locomotives are generally started at full gear, admitting steam 
for nearly the entire stroke and then exhausting it at relatively high 
pressure. This wasteful use of steam is necessary to furnish the 
power needed in starting a train. After the train is under way, 
less power is required per stroke and the link is gradually moved 
toward mid gear or "hooked up" by the engineer, thus hastening 
the cut-off; the expansion is increased and the power is reduced in 
proportion to the load. 



58 VALVE GEARS 

As the cut-off is changed, it is desirable to maintain an approxi- 
mately equal cut-off at each end of the cylinder ; this can be secured 
in the Stephenson gear by properly locating the saddle pin and the 
reverse shaft. When used without a rocker, as in Fig. 52, the sad- 
dle pin should be on the arc of the link or slightly ahead of it. When 
used with a rocker, the saddle pin should be behind the link arcs 
and, in order to give symmetrical action for both forward and back- 
ward running, it should be opposite the middle of the arc, that is, 
equally distant from each link pin. 

Zeuner Diagram for Stephenson Gear. The Stephenson link can 
not be designed directly from the Zeuner diagram, but a systematic 
investigation can be made by using a wooden model of the proposed 
link. This can be mounted on a drawing board, and the effect of 
.changing the position of pins and the proportions of rods and levers 
can be determined without difficulty. By a system of trials, a com- 
bination can be found best suited to obtain the desired results. 
Moreover, a model makes it possible to measure directly the slip of 
the link block along the link. This slip should be kept as small as 
possible to prevent rapid wear. It can be controlled to some extent 
by properly locating the link pins, by avoiding too short a link, and 
by choosing a favorable position for the reverse shaft. 

The Zeuner diagram for a Stephenson gear embodies all of the 
principles of the Zeuner diagram for a simple valve, with certain 
additional ones which, while comparatively simple, sometimes cause 
confusion. It is only necessary to remember that there are two 
eccentrics and that their combined action is the same as one virtual 
eccentric; also, that in passing from a long to a short cut-off with 
open rods, the lead increases, hence the path of the virtual eccentric 
center must be a curved one. A practical example will make the 
construction of such a diagram clear. 

Example. Given a maximum valve travel of o\ inches, steam lap 

r> 

1 inch, lead at full gear *$ inch, and — equal to \. Find the valve travel 

Li 

for 60 and 80 per cent cut-off, respectively. 

Solution. Construct the valve travel circle A B C D, Fig. 56, having 
• a diameter of 5| inches. (The scale of the drawing is exactly i size.) Draw 
the lap circle T U V W and lay off the full gear lead S T. Lay off the angles 
FOB and Q O D equal to the angle through which the eccentrics must be 
turned in order to displace the valve by an amount equal to the lap plus the 
lead at full gear; or with slight error draw a perpendicular to A C through 



VALVE GEARS 



59 



the point S and where it cuts the maximum valve circle, as at P and Q, will 
be the centers of the eccentrics sought. Two points of the locus of the virtual 
eccentric center have thus been established. In order to draw the locus, the 
amount of lead at mid gear must be known. By the construction of Fig. 53, 
it was shown that in shifting the link from the full gear position G l H l to 
the mid gear position G 2 H 2 . the lead was increased by the amount G X J, which 
can be measured directly from the drawing. In this problem assume that the 









B 


__^60% 


CIST-OFF 








K^~~~r 










F 






*jC&0 % CUT-OFF 




___y 4 










MX- — — _\_ 
















sK^^ 




T 




A 


ye 


/ 


fi Q i 


T 






v\ 




A- \G G 


HEAD 
END 












o 




CRANK 
END 












w 












L . y 












77 








tfT\^^ 














~D~~. 





Fig. 56. Zeuner Diagram for Stephenson Gear 

length of the eccentric blades is known and that by a construction similar to 
that in Fig. 53, the lead at mid gear was found to be f inch, or, in other 
words, in passing from full to mid gear the lead was increased -^ inch. Knowing 
the lead at mid gear, lay off the distance TR equal to f inch. The locus of the 
virtual eccentric center must pass through P, Q, and R and have its center on 
the line A C extended. By trial, we find such a center and such a radius that 
the arc when drawn will pass through the points P, Q, and R. This arc is the 
locus of the virtual eccentric center when dealing with the head-end events. 
To find the events for the crank end, construct a similar arc on the right of the 



60 



VALVE GEARS 



vertical line B D. To obtain the valve travel for 60 per cent cut-off, first 
determine the crank position in the usual manner by locating the line Z, 

, . , R 

remembering that — = \. Where this line cuts the lap circle, as at X, draw 

Li 

a tangent to the lap circle and extend it until it cuts the arc P RQ at M. 
M is then the radius of the valve travel circle for 60 per cent cut-off. Con- 
struct the valve travel circle I J K L with a diameter of 3^ inches, the 
required valve travel. In like manner, establish the point N, which deter- 
mines the valve travel circle E F G H f or 80 per cent cut-off, the diameter of 
which : .s 4| inches. By this same procedure, the valve travel for any cut-off 



4\C 




/OLD 
Fig. 57. Valve Ellipse Diagram for Studying Valve Action 



may be obtained. Having the valve travel circle established, all the events 
of the stroke may be found as has already been pointed out in the study of 
the Zeuner diagram. 

Valve Ellipse Diagram. The valve ellipse diagram furnishes 
another method for studying the valve action, aside from that fur- 
nished by the Zeuner diagram. The valve ellipse has been used a 
number of years as a means for representing the relative positions 
of the valve and the piston. 



VALVE GEARS 



61 



The principle of its construction as applied to the arrangement 
of valve and rods, as shown in Fig. 55, is to draw lines at right angles 
to each other, one representing the travel of the piston, the other 
that of the valve. Thus, in Fig. 57, draw the circle ABCD, hav- 
ing a diameter equal to the stroke of the piston drawn to a predeter- 
mined scale. This circle represents the path of the crank pin center. 
Divide this circle into any number of equal divisions, in this case, 
twelve, at points 1, 2, 3, etc. It is evident that if a line be drawn 
from any one of these points, as 2, perpendicular to the line AB, 
that, neglecting the angularity of the connecting rod, the distance 
Am would represent the displacement of the piston as the crank 
moved forward from A. To allow for the angularity of the connect- 
ing rod, take a radius equal to the length of the connecting rod drawn 




Fig. 58. Valve Ellipse Diagram Showing Information to be Obtained from its Analysis 



to the same scale as that of the circle ABCD and sweep the arcs 
from the points 1,2,3, etc., with the center on the line A B produced. 
Now representing the path of motion of the valve by the line H F, 
drawn perpendicular to A B, the eccentric position I — which is 
located at the angle (90 — 0) degrees behind the crank— is, for the 
sake of convenience in constructing the ellipse, located at J. 
Having drawn the valve travel circle E F G H, begin at J and lay 
it off into the same number of equal parts as was done in the case of 
the crank circle. For the crank position A, the corresponding 
eccentric position is J, and hence, by projecting a vertical line and 
a horizontal line from A and J, respectively, the point a is located. 
In the same manner,^ the points b, c, d, etc., are located, thus com- 
pleting the construction of the ellipse. The ellipse may have 



62 VALVE GEARS 

different inclinations to the reference line, depending on conditions. 
This difference will be noted in comparing Figs. 57 and 58. 

Thus far the discussion has dealt only with the construction of 
the ellipse. It is now proposed to point out what information may 
be obtained from the valve ellipse and for the sake of clearness, 
another figure is shown. After constructing the ellipse or having 
obtained it directly by an instrument specially constructed for. the 
purpose ; draw the reference line C in Fig. 58. Tangent to the ellipse, 
draw the lines F and G parallel to C. The distance between the 
lines F and G represents the travel of the valve. Midway between 
F and G draw the line D, the center line of the extreme travel of 
the valve. Assume that the valve is an ordinary plain slide valve 
having If inches steam lap and the zero exhaust lap, or line and 
line. Draw the lines P and Q 1| inches on each side of the center 
line D, and where these lines cut the ellipse determines the points 
where admission and cut-off occur for the two ends of the cylinder, 
as indicated in Fig. 58. Since there is no exhaust lap, the point 
where the line D cuts the ellipse gives compression and release for 
the two ends of the cylinder. In this case, the compression occurs 
on the head end at the same time that release occurs on the crank 
end, and vice versa. If the valve be given exhaust lap, it would be 
laid off in the same manner as the steam lap. Draw the lines Y and 
Z tangent to the ellipse and perpendicular to the reference line C. 
The distance between these two lines represents the length of the 
stroke of the engine drawn to scale. To find the per cent of the 
stroke at which any event occurs, it would be only necessary to 
drop a perpendicular to the center line D from the point on the 
ellipse corresponding to the event under consideration and obtain 
the percentage as previously explained. If the width of the steam 
port be known, it would be laid off from the lines P and Q toward 
the lines F and G as indicated. Assuming admission on the head 
end to occur as marked on the line P, it is evident from the portion 
of the curve contained between the lines F and P that at the begin- 
ning of the stroke the steam port was opened rather quickly and 
that cut-off occurred by the port being closed very slowly. During 
this time, the piston moved approximately three-quarters of the 
stroke. There being no exhaust lap or inside clearance, release 
occurred when the valve reached its central pc sit ion. At the same 



VALVE GEARS 63 

time that head-end release took place, compression began on the 
crank end, then followed crank-end admission, cut-off, release, and 
head-end compression, in regular order. 

The valve ellipse has been largely used by steam railroad engi- 
neers and, as a result of the demand for such information as can be 
obtained from a consistent study of it, several devices have been 
invented for taking the ellipse directly from the engine. These 
devices consist of a drum the circumference of which is made propor- 
tional to the stroke of the engine. A sheet of paper is held on this 
drum by means of clips somewhat in the same manner as are found 
on the drums of steam engine indicators. This drum is mounted 
on a frame and when in use is placed in a convenient position above 
the crosshead or on the steam chest in such a position that its axis 
of rotation is perpendicular to the direction of the motion of the 
valve. The drum is rotated by means of a cord connection with the 
crosshead. Attached to the apparatus is a pencil which receives 
the same motion as that of the valve by means of a connection with 
the valve rod. Hence by 
the combination of the two 
movements, that is, of the 
drum moving with the pis- 
ton and that of the pencil 
moving with the valve, the 
valve ellipse diagram is 

drawn ^ s * 5 ^' Gooch Link Motion 

From the study of the Stephenson gear, it is obvious that it is 
very flexible, and that it is readily adjusted to all irregularities of 
operation. Great care must be taken in its design in order that it 
may properly perform its work. Owing to the large number of 
parts and the size of same on large engines, it frequently gets out of 
alignment, its parts wear considerably and, on locomotives, the lubri- 
cation is sometimes difficult. On this account, it requires frequent 
attention in order that the best results may be obtained. All things 
considered, it is doubtful whether any other reversing gear gives as 
good a steam distribution as does the Stephenson gear when it is 
properly adjusted and operated. 

Gooch Link. The Gooch link, illustrated in Fig. 59, has been 
extensively used on European locomotives, although it is gradually 




64 VALVE GEARS 

being replaced by a type of valve gear known as the Walschaert, 
which will be described later. 

The Gooch link has its concave side turned toward the valve 
instead of toward the eccentric. The radius of curvature of the link 
is equal to A B, the length of the radius rod. The link is stationary 
and the link block slides in the link. The engine is reversed by 
means of the bell-crank lever on the reverse shaft E which shifts 
the link block instead of the link, as is the case with the Stephenson. 
The link is suspended from its saddle pin M, which is connected by 
a rod to the fixed center F so that the link can move forward and back 
as the eccentricity is changed, or it can pivot about its saddle pin as 
the eccentrics revolve. 

Since the radius of the link arc is equal to A B, it is apparent 
that the block can be moved from one end of the link to the other, 
that is, from full gear "forward" to full gear "back" without moving 
the point A, which is on the end of the valve rod. The lead then is 
constant for all positions of the block. The gear is more complicated 
than the Stephenson and requires nearly double the distance between 
shaft and valve stem. 

RADIAL TYPE OF VALVE GEAR 

In general, it would be desirable to have precisely similar steam 
distribution at each end of the cylinder, and it would often be of* 
great advantage with a gear like the Stephenson if the cut-off could 
be shortened without changing any other event of the stroke. A 
Stephenson gear can be made to maintain equality of lead for both 
ends of the cylinder as the cut-off is shortened, but we have seen that 
in so doing, the lead of both ends is either increased or diminished 
according as the link is arranged with "open rods" or "crossed rods." 
Moreover, the compression is hastened by bringing the link to mid 
gear, all of which in many instances is undesirable. 

This disadvantage of the Stephenson link motion led to the 
design of the so-called "radial valve gears," many of which are so 
complicated as to be impracticable, but all of which obtain a fairly 
uniform distribution of steam. 

Hackworth Gear. The essential features of the Hackworth 
gear are indicated in outline in Fig. 60. In this figure S is the center 
of the shaft, and the eccentric E is set 180 degrees from the crank 



VALVE GEARS 



65 



SH. At the right-hand end of the eccentric rod EA is pivoted a 
block which slides in a straight, slotted guide. The guide remains 
stationary while the engine is running, but can be turned on its axis 
P to reverse the engine or to change the cut-off. P is a pivot which 
is located on a horizontal line through $ in such a position that 
D P is equal to EA. If these two distances are equal, A will coin- 
cide with P when the crank is at either dead point and the slotted 
guide may be turned from "full gear forward" (as shown in the figure) 




Fig. 60. Diagram of Hackworth Radial Valve Gear 

through the horizontal position to "full gear back" (as shown by 
the line BL) without moving the valve. It will be observed, there- 
fore, that the leads are constant for all positions of the guide. The 
valve rod running upward from C connects with the valve stem, 
which it moves in a straight line. The valve stem is made just 
long enough to equalize both leads and, if the point C has been prop- 
erly chosen, the two cut-offs will be very nearly equal for all grades 
of the gear. 

A somewhat better valve action is obtained by slightly curving 
the slotted guide, with its convex side downward. This gear is 
sometimes used on marine engines and on small stationary engines. 



VALVE GEARS 



Marshall Gear. The most objectionable feature of the Hack- 
worth gear is, perhaps, the slotted guide, for the sliding of the block 
causes considerable friction and wear. The Marshall gear, shown 
in outline in Fig. 61, is designed to eliminate this feature. The point 
A moves in the desired path by swinging on the rod FA about F 
as a center. While the engine is running, the lever F P remains 
stationary, but can be turned on its axis P to reverse the engine or 
to change the point of cut-off. The pivot P is located precisely as 




Fig. 61. Diagram of Marshall Radial Valve Gear 

in the Hackworth gear, and the lever F P can be turned from "full 
gear forward" (as shown in the figure) to "full gear back" (as shown 
by the line BP), intermediate positions giving different cut-offs the 
same as with the Hackworth gear. Since F A is made equal to F P, 
the point A will always swing through P no matter where F may be 
and will coincide with P when the engine is on dead center. The 
leads for all positions of the gear, therefore, will remain constant, 
as in the preceding case. 

The Marshall gear is sometimes made with the point C located 
on the right of A, on the line EA produced. In this case, if the 
same kind of valve is to be used, the eccentric E must move with 



VALVE GEARS 67 

the crank instead of 180 degrees from it. The Marshall gear is fre- 
quently used on marine engines, the one eccentric being simpler than 
the two required by the Stephenson. 

Joy Gear. The Joy radial gear, Fig. 62, is perhaps the most 
widely known, and is certainly one of the best radial gears. It is 
frequently used on marine engines and on some English locomotives. 
No eccentrics are used, the valve motion being taken from C, a point 
on the connecting rod. H is a fixed pivot supported on the cylinder 
casting. The lever ED has a block pivoted at A, which slides back 
and forth in a slotted guide, having a slight curvature, the concave 
side being toward the right. The guide and the lever P F are fas- 
tened to the reverse shaft P and, by means of a reverse rod leading 
off from F, can be turned from full gear forward, as shown, to full 
gear back, when the pin F moves over to the position B. Motion 
is transmitted to the valve stem bv means of the radius rod E G. 



Fig. 62. Diagram of Joy Radial Valve Gear 

The proportions are such that when the crank is on either dead 
point, the pivot of block A coincides with P, so that the curved 
guide may then be set in any position without moving the valve; 
therefore the leads are constant. This gear gives a rapid motion to 
the valve when opening and closing and a more nearly constant 
compression than the Stephenson gear, and the cut-off can be made 
very nearly equal for all positions of the gear. Its many joints 
cause wear and its position near the crosshead makes a careful inspec- 
tion of the crosshead and piston rather difficult while the engine is 
running. 

Walschaert Gear. The Walschaert gear, Fig. 63, stands today 
as the best representative of the radial gear type. It has for many 



68 



VALVE GEARS 



years been very largely used on all the important European rail- 
roads. It has been used considerably in England and at the present 
time is being applied to a great many locomotives in America. 

Analysis of Valve Motion. When the Walschaert gear is used, 
the valve receives its motion from two distinct sources, namely, 
from the crosshead and from the eccentric crank. In Fig. 64 the 
various parts of the gear are named. The crosshead connection 
gives to the valve a movement equal to the lap plus the lead, at the 
extremities of the stroke, when the eccentric crank is in its mid- 
position. The eccentric crank leads the main crank by an angle 
of 90 degrees for a valve having external admission, and follows the 




E3- 



Fig. 63. Diagram of Walschaert Radial Valve Gear 



main crank by 90 degrees for an internal admission valve. Locat- 
ing the eccentric crank exactly 90 degrees in advance or behind the 
main crank is one of the necessary adjustments of the Walschaert 
gear. It is evident, therefore, that if an eccentric rod of proper 
length be attached to the eccentric crank and the valve through 
proper means, when the engine is on dead center, the valve 
would be in mid-position. However, in order to have economic 
operation, it becomes necessary to have some lead at dead center 
positions, hence the valve must be displaced by an amount equal 
to the lap plus the lead. Since the eccentric crank must be 90 
degrees from the main crank, some other means must be used to 



VALVE GEARS 



69 



obtain the proper displacement and the method of accomplishing 
this on the Walschaert gear is one of its most distinguishing features. 
An attachment is made between the crosshead and the valve stem 
by means of a lever known as the combination lever, or, as shown in 
Fig. 64, the lap and lead lever. 

In order to obtain the proper displacement of the valve when 
the engine is on dead center, the attachment of the combination 
lever to the crosshead and to the valve rod must bear a definite ratio 
to the stroke and valve travel. In other words 



or 



S:tasL:V 



S 



in which S is stroke of piston in inches ; t is twice the sum of the lap 
plus the lead in inches; L is distance between the crosshead connec- 



\rl/rr///G ARM 



CENTER OFL/fT SHAFT 




Fig. 64. Diagrammatic Analysis of Walschaert Valve Gear Action 



tion and that of the radius lever in inches; and V is distance between 
the connection of the radius lever and that of the valve stem in 
inches. The above expression holds good for either an inside or an 
outside admission valve. 

When using an inside admission valve, the connection between 
the radius rod and the combination lever is made above the valve 
stem connection as shown in Fig. 64, that shown in Fig. 63 beinj; 
the arrangement for an external admission valve. 



70 VALVE GEARS 

Link Motion. We have thus far traced the movement of the 
valve, taking into consideration the crosshead and eccentric crank 
connection and omitting for the sake of clearness the link connection. 
It will be noted that the link is pivoted at the center. The link block 
is raised or lowered by means of the reverse lever and bell crank. 
The link block is connected to the radius rod, which has a length 
equal to that of the link; hence, when the engine is on either dead 
center, the link block can be raised from one extreme position to the 
other without moving the valve. Therefore this gear, if properly 
constructed, gives a constant lead for all positions of the reverse lever. 
The proper construction, suspension, and attachment of the link 
to its allied parts is a very important matter and one rather diffi- 
cult to accomplish. The proper location of the attachment of the 
link to the eccentric rod gives the designer a great deal of trouble, 
in obtaining the desired action of the valve. In locating the longi- 
tudinal position of the link fulcrum, consideration must be given to 
the length of the eccentric rod, which should have a minimum length 
of three and one-half times the throw of the eccentric and should 
be made as long as the existing conditions will permit. It should 
be so located that the radius and eccentric rods are approximately 
of equal length. The point of connection between the link and 
the eccentric rod should be as near the center line of motion of the 
connecting rod as possible, making due allowance for the angularity 
of the rods. To accomplish this, it often happens that the throw 
would be excessive. In such cases, a compromise is necessary, the 
point of connection being raised above the center line of motion as 
the case demands. It has been found in designing this gear that these 
considerations require shifting the eccentric crank from one to two 
degrees, thus making the angle between the main crank and the eccen- 
tric crank 91 degreesor 92 degrees instead of 90 degrees, as theoretically 
required. The angle being increased by such a small amount does 
not affect the movement of the valve to any appreciable extent. 

Adjustment of Gear. From the foregoing brief remarks, it is 
to be noted that in order to secure the best results, the design of 
the Walschaert gear requires very accurate work. Xo hard and 
fast rules can be laid down as how to secure the best design, for each 
case presents different problems. The best way to secure required 
results is to try out the design on a model. 



VALVE GEARS 71 

The American Locomotive Company gives the following sug- 
gestions for adjusting the Walschaert valve gear: 

(1) The motion must be adjusted with the crank on the dead center 
by lengthening or shortening the eccentric rod until the link takes such a 
position as to impart no motion to the valve when the link block is moved 
from its extreme forward to its extreme backward position. Before these 
changes in the eccentric are resorted to, the length of the valve stem should 
be examined as it may be of advantage to plane off or line under the foot of 
the link support which might correct the length of both rods, or at least only 
one of these should need be changed. 

(2) The difference between the two positions of the valve on the for- 
ward and back centers is the lead and lap doubled and can not be changed 
except by changing the leverage of the combination lever. 

(3) A given lead determines the lap or a given lap determines the lead, 
and it must be divided for both ends as desired by lengthening or shortening 
the valve spindle. 

(4) Within certain limits, this adjustment may be made by shorten- 
ing or lengthening the radius bar but it is desirable to keep the length of this 
bar equal to the radius of the link in order to meet the requirements of the 
first condition. 

(5) The lead may be increased by reducing the lap, and the cut-off 
point will then be slightly advanced. Increasing the lap introduces the oppo- 
site effect on the cut-off. With good judgment, these qualities may be varied 
to offset other irregularities inherent in transforming rotary into lineal motion. 

(6) Slight variations may be made in the cut-off points as covered by 
the preceding paragaph but an independent adjustment can not be made 
except by shifting the location of the suspension point which is preferably 
determined by a model. 

Zeuner Diagram for Walschaert Gear. The Walschaert gear 
may be examined by the aid of a Zeuner diagram to the same limited 
extent as the Stephenson. The construction of the Zeuner for a 
Walschaert gear is somewhat easier than for the Stephenson because 
the locus of the virtual eccentric centers lie on a straight line, due to 
the constant lead. For example, take a maximum valve travel of 5J 
inches, a lap of 1 inch, and a lead of ^ inch. In Fig. 65 (scale of 
drawing is exactly three-fourths size), the valve travel circle is 
ABCD, the lap circle ILMN. Lay off the given lead IF 
& inch and through the point F erect a perpendicular line cutting 
the circle ABCD at H and E, thus locating the two eccentric posi- 
tions. Since there is a constant lead for any valve travel, the line 
H F E becomes the locus of the virtual eccentric centers. Assum ■ 
ing a cut-off of 80 per cent, locate the line K and at J, the point 
where this line cuts the steam-lap circle, erect a perpendicular line 



VALVE GEARS 



and extend it until it cuts the line H F E at G. The point G is the 
extremity of the valve travel circle for 80 per cent cut-off, the radius 

B 



80 % CUT-OFF 




Fig. 66. Application of Walschaert Valve Gear to a Passenger Locomotive 

of which will be G, 2 ^ inches. In like manner, the valve travel 
can be obtained for any other point of cut-off. 



VALVE GEARS 



73 



Dimensions of Walschaert Gear Parts. For an engine such as is 
shown in Fig. 66, an approximate value of the various rods and levers 
may be taken as follows. By referring to Fig. 64 the location of the 
various parts can be determined more readily than in Fig. 66. 

Main rod 8-0" Radius rod 3-10" 

Eccentric crank . l'-2" Lap and lead lever (total) .... 3'- 0" 

Eccentricity.... 6|" Lap and lead lever connector.. V- 2" 

Eccentric rod . . . 4'-6" Crosshead arm 1'- 0" 

Linkarc l'-lO" Stroke 2'- 0" 

DOUBLE VALVE GEARS 

It has been shown that a plain slide valve under the control of 
a gear, that gives a variable cut-off, such as a shifting eccentric or a 
link motion, will not give a satisfactory distribution of steam at a 
short cut-off owing to excessive compression, variable lead, or early 
release. These difficulties are overcome in a measure by the use of 
the radial gear; and also by the use of two valves instead of one. 




Fig. 67. Section of Meyer Double Valve 



The main valve controls admission, release, and compression; the 
other, called the cut-off valve, regulates the cut-off only, which may 
be changed without in any way affecting the other events of the 
stroke. This cut-off valve, sometimes known as the riding cut-off 
valve, may be placed in a separate valve chest, or it may be placed 
on the back of the main valve. 

Meyer Valve. The most common form of double valve gear 
is the Meyer valve, Fig. 67. The cut-off valve is made in two parts 
and works on the back of the main valve. The two parts are con- 
nected to a valve spindle with a right-hand and a left-hand thread, 
so that their positions may be altered by rotating the valve spindle. 

A swivel joint is usually fitted in the valve spindle between the 



74 



VALVE GEARS 



steam chest and the head of the valve rod, and the valve spindle is 
prolonged into a tail rod which projects through a stuffing box on the 
head of the steam chest, Fig. 68. The end of this tail rod is square in 
section and reciprocates through a small hand wheel, by means of 
which it can be rotated while the engine is running, whatever the 
position of the valve may be. 

Each valve is under the control of a separate eccentric. The 
eccentric which moves the main valve is usually fixed, while the 
cut-off valve eccentric may be under the control of a governor. Since 
a slight compression is desired, the main valve is set to give late cut- 
off and this will also give late release and late compression, and allow 

a wide range of cut-off for the cut- 
off valve. With this gear, lead, 
release, and compression are en- 
tirely independent of the ratio of 
expansion, and the cut-off is much 
sharper, because the cut-off valve, 
when closing the ports, is always 
moving in a direction opposite 
to that of the main valve. The 
valve may be designed by means 
of the Zeuner diagram. 

Design by Zeuner Diagram. 
Let us design a Meyer valve hav- 
ing an eccentricity of 2 inches. 
Let the eccentricity of the cut-oft 
valve be 2\ inches and the relative travel of the cut-off valve in rela- 
tion to the main valve be 3 inches. This will make the relative motion 
of the cut-off valve equivalent to the travel of a plain slide valve 
with an eccentricity of 1J inches. Let the outside lap on the main 
valve be f inch, the lead A inch, the compression 5 per cent of the 
stroke, and let the ratio of the length of the crank to connecting rod 
be 1 to 6. 

In Fig. 69, draw X Y, the main valve travel, equal to 4 inches. 
Lay off X D equal to 5 per cent of 4 or 0.2 inches, and with a radius 
of 12 inches, and the center on Y X produced, draw the arc D H K. 
H K is the crank position at compression on the head end ;C K 0, 
the crank position at compression on the crank end, is found in a 




Fig. 68. 



Stuffing Box and Valve Spindle 
of Meyer Gear 



VALVE GEARS 



75 



similar manner. Lay off I equal to the lap plus the lead, and draw 
the valve circle for the main valve through / and with a diameter 
equal to its. eccentricity of 2 inches. To do this, take a radius equal 
to 1 inch, and draw arcs from I and as centers that shall intersect 
at B. B is the center of the valve circle and B E is the eccentricity, 
2 inches. With E as a center, and with a radius equal to half the 
relative travel of the cut-off valve (in this 1J inches) draw an arc. 




Fig. 69. Zeuner Diagram for Meyer Valve Gear 



With as a center and with a radius equal to 2\ inches, the eccen- 
tricity of the cut-off valve, draw another arc intersecting the first one 
at F. On F as a diameter, construct a valve circle. This valve 
circle will represent the absolute motion of the cut-off valve, inde- 
pendent of the motion of the main valve. This circle then will show 
the displacements of the cut-off valve from the center of the steam 
chest. W r ith E as a center and with a radius equal to F 0, draw an 
arc, and with as a center and with a radius equal to E F, draw 
another arc intersecting the first at G. On G as a diameter, con- 
struct a valve circle. This circle will then represent the travel of the 
cut-off valve moving on the main valve. That is, it will represent the 



76 VALVE GEARS 

displacements of the cut-off valve from the center of the main valve. 
This circle is not, properly speaking, a valve circle, and G is not an 
eccentricity, but simply represents the relative motion of the two 
valves. This can be proved by analytical geometry, but an inspection 
of the figure shows that this must be true. 

Draw the crank line C at any position, cutting the valve circles 
at T and U and V. V represents the absolute displacement of the 
cut-off valve, that is, from the center of the steam chest, and T 
represents the displacement of the main valve. The relative dis- 
placement of the cut-off valve, that is, from the center of the main 
valve, will be the difference between V and T, since both valves 
are moving in the same direction. By careful measurement it will be 
found that U = T—0 V, and any arc as U on the auxiliary 
circle U G will correctly represent the displacement of the cut-off 
valve from the center of the main valve at the corresponding crank 
angle. 

In Fig. 70 are shown H K the crank angle at head-end compres- 
sion, C K the crank angle at crank-end compression, the main valve 
circle, and the auxiliary circle, all of which have been transferred 
from Fig. 69. In order to avoid confusion, the construction lines 
and all lines not essential to the figure are omitted. 

Lay off on Fig. 70, I equal to the outside lap f inch and draw 
the head-end lap circle H E 0. It will intersect the valve circle for 
the main valve at L and M. Draw H through L, representing the 
crank position at admission (head end) and M H through M show- 
ing the crank position at cut-off. This gives the greatest possible 
cut-off. The cut-off valve may be set to give a much earlier cut-off 
than this, but of course, a later setting would be of no avail, for the 
port would be closed by the main valve at this angle. The crank 
line M H cuts the auxiliary circle at A T i so that Ai (1 Jf inches) 
is the clearance of the cut-off valve. That is, the edge of the cut-off 
valve must be set 1 Jj inches from the edge of the main valve port 
in order to cut off at this crank angle. The full lines of Fig. 66 show 
the cut-off valve placed in this position. 

The intersection of II K with the lower valve circle gives the 
inside lap at the head end of the cylinder. This line comes so nearly 
tangent to the valve circle that the intersection can be determined 
only by dropping a perpendicular to H K from E 2 . This cuts the 



VALVE GEARS 



77 



circle at P, and P equals the head-end inside lap ^ inch, and H E I 
represents the corresponding lap circle. 

The crank position at compression on the crank end is C K, 
which cuts the upper valve circle at A r 2 , giving an inside lap for the 



Eu 



M/N/MUV 

cur-orr /5%. 



Ji 



yy, 



X.K 



H.A. 



0-^j 



M. 



C.A 



'*£» 



HH> 



Fig. 70. Zeuner Diagram Showing Additional Factors in Design of Meyer Valve 



crank end, of N 2 , H inch. To make this intersection more appar- 
ent, the perpendicular may be drawn from E\, as previously explained. 
Suppose that it was required that the minimum cut-off should 
be 15 per cent. Find the crank position at 15 per cent of the stroke 
in the same manner as the crank position was found at compression. 
Produce this line through until it cuts the auxiliary circle at S. 
Then S equals the required lap §f inch for the cut-off valve in 
order to cut off at 15 per cent of the stroke. The dotted lines in 
Fig. 67 show the cut-off valve drawn in this position. 



78 



VALVE GEARS 



For a valve of this sort, the cylinder port should be 1J inches 
wide and the valve port 1 inch wide. Fig. 67 shows this valve laid 
out to scale, but as this process is in all respects similar to that 
described for laying out a plain slide valve, it will not be described 
in detail. 

Shifting Eccentric Valve Gear. In addition to the valve gears 
already described, there is another class which receives a very large 
application, particularly in small and medium-power high-speed 
engines. This class of gear is what may be termed the shifting 




Fig. 71. Diagram Showing Action of Straight-Line Type of Shaft Governor 

eccentric gear. The valve itself is the ordinary flat slide or piston 
valve, and the valve stem, eccentric rod, and eccentric are the same 
as used in the common arrangement. The difference between the 
fixed and the shifting eccentric lies in the method of attaching the 
latter to the shaft and in the mechanism provided to move this 
eccentric from one position to another across the shaft. The general 
arrangement is illustrated in Fig. 71, which represents that used on 
the straight-line engine. 

In this, is the fixed pivot of the eccentric lever E, and C is the 
eccentric. The pin H of the eccentric lever is connected through a 
link to a leaf spring and through the other to a weighted lever B, as 
shown. When the engine is running, the position of the weight B 



VALVE GEARS 



79 



changes under different speeds and loads, and this change in position 
is transmitted to the eccentric. Since is a fixed pivot, any motion 
of the eccentric lever E or eccentric C must be around as a center. 
Consequently, when the eccentric position changes, its center will 
move in a path which is an arc of a circle with a center at 0. The slot 



7\ 

i\ \ 



r 






W 



\ \ 
\ \ 
\ \ 

\{ 



X 



■<f 






yp' 



— A_ -- / 



Fig. 72. Zeuner Diagram for Shifting Eccentric Valve Gear 



in the eccentric is provided so as to enable it to move across the shaft 
to whatever position may be desired. 

Two things are to be noticed for different eccentric positions. 
The first is that the eccentricity becomes less as the eccentric center 
moves along its path toward the shaft. The second is that the angular 
advance increases under this same condition. The first effect is to 
decrease the valve travel and the second to increase the angular 



SO VALVE GEARS 

advance. The effect of the shifting of the eccentric on the motion of 
the valve is a combination of these two changes. 

Fig. 30 shows the effect of changing the angular advance, and 
Fig. 31 shows the effect of changing the valve travel. The effect of 
both these changes acting together is shown in Fig. 72, which is a 
Zeuner diagram for a shifting eccentric valve gear. 

Analysis of Zeuner Diagram. In Fig. 72 the full lines represent 
eccentric and crank positions at the point of maximum cut-off, and 
the dotted lines represent their positions corresponding to some 
earlier cut-off. Ai is the crank position at admission; C\ is its position 
at cut-off; Bi is its position at release; and Di is its position at com- 
pression. Pi and P 2 are the corresponding positions of the eccentric 
center, the subscripts referring to the maximum cut-off and to the 
earlier cut-off, respectively. The radius R is used to draw the path 
of the eccentric center and has the same length as the distance from 
the fixed eccentric lever pin to the center of the eccentric. The steam 
lap in the figure is XL and the exhaust lap is X N. The construction 
is made for the head end only, to avoid confusion due to the large 
number of additional lines required for that of the crank end. These 
laps remain the same for all positions of the eccentric. 

Each of these two diagrams is made in exactly the same way as 
the ordinary Zeuner diagram and, if given the necessary data, the 
construction of the combined diagram should give no trouble. One 
point to bear in mind in drawing a Zeuner for this kind of valve gear 
is that the eccentric center for any position of the gear will lie some- 
where along the arc described with the radius R. 

An inspection of Fig. 72 shows that all events occur earlier with 
the earlier cut-off. However, they do not all continue for the same 
period, as was found to be the case when the angular advance alone 
was changed. This is because the valve travel is changed with the 
angular advance. The combined effect is that admission is advanced 
very slightly, cut-off is advanced a considerable amount, and release 
and compression are each advanced a moderate amount. Since the 
cut-off advances a greater amount than the release, the result is a 
greater expansion at earlier cut-off, which is more economical, within 
reasonable limits, in the use of steam. Also the increase in compres- 
sion up to a certain point will make the engine run more smoothly by 
cushioning the piston better on the dead centers. Another effect of 



VALVE GEARS 



81 



earlier cut-off is a decrease of lead from that at maximum cut-off. 
This may or may not be an advantage, depending on the engine 
speed, construction of steam ports, amount of clearance, etc. 

Thompson Automatic Valve Gear. The Thompson automatic 
valve gear, commonly known as the "Buckeye", belongs to the 
general class of double valve gears. Its principle of action involves 
certain ingenious points which make its study very interesting. 

Two styles of valves of this type have been developed by the 
Buckeye Engine Company, namely, a flat valve and a round or piston 




Fig. 73. 



Section of Cylinder and Valve-Gear Mechanism of Thompson 
Automatic Valve Gear 



valve. While the essential features of the two valves are the same, 
the piston valve, illustrated in Fig. 73, is the simpler of the two and 
represents the latest practice. The cut-off valve A moves inside 
of the main valve B, and live steam entering at C passes through 
the cut-off ports D D in the main valve and is admitted to the 
cylinder, as these ports are alternately brought into coincidence with 
the cylinder ports E E y as shown by the arrows on the left. The 
exhaust steam is discharged at the ends of the main valve and does 
not come in contact with the valve except at the ends. It will be 
noted that the construction is such that the valve is at all times 
balanced. 

The valve stem F of the cut-off valve passes through the hollow 
stem G of the main valve. Packing rings are used on both valves 




3 

pq 
o .§ 

-a o 

o, 

a 
< 



> s 



I* 



£ 

o 



VALVE GEARS 83 

to insure a steam-tight connection, and bridges are provided to afford 
a proper bearing for the rings in passing over the ports. 

Fig. 74 shows the valve gear as applied to the engine. The 
operation of the gear can be better understood by referring to the 
line drawing, Fig. 75. The crank A C is shown on the head-end dead 
center and running over. The eccentric D connects to the main 
valve stem through the eccentric rod D M, the joint M being guided 
by the rocker arm H M, pivoted to the engine frame at H. The 
cut-off valve is operated from the eccentric E by the eccentric rod 
E F, and the rocker arm F K N is pivoted to the rocker arm M H at 




Fig. 75. Diagram Showing Operation of Thompson Automatic Valve Gear 

the point K. In the compound rocker arm, the arms M K, K H, 
N K, and K F are all equal. On account of the valve under dis- 
cussion being one having internal admission, it will be noted that the 
eccentric of the main valve follows the crank instead of preceding it, 
as is found in most cases. 

Starting from the head-end dead center, suppose the crank, and 
likewise the eccentric, to turn through a small angle 4>. This move- 
ment will cause the point M and the main valve to move to the left, 
a distance which we will call x, and the pivot point K will also move 

to the left a distance -^-. Now if F be considered as a fixed point, the 

-r X 

movement of K, equal to — , causes the point N, and consequently the 

Li 

cut-off valve, to move to the left a distance x. The point F is not 
fixed, for while M is moving a distance x to the left, the rocker arm 
F K N, being pivoted to the rocker arm H K M at the point K, will 
cause the point F to move to the left (due to the rotation of the eccen- 



84 VALVE GEARS 

trie E) a distance which we will call y. This movement of F would 
cause N to move to the right a distance equal to y, provided the 
point K were stationary. Thus it will be seen that the point N and 
the cut-off valve are given a movement which is the resultant of two 
motions and is equivalent to x— y; and the relative movement of 
the two valves would be x-(x-y)=y. But y is the motion which 
would be given the cut-off valve by the eccentric E, independent of 
the other mechanism; i.e., the construction is such that the cut-off 
valve moves on the main valve in much the same manner as an 




Fig. 76. Indicator Diagram, Showing Effect of Application of Thompson Gear 

ordinary plain slide or piston valve moves over stationary ports 
when connected to a constant throw eccentric through a reversing 
rocker arm. 

The governor in moving the cut-off eccentric simply causes it 
to turn about the shaft, thus cutting off the steam earlier or later, 
according as the eccentric is advanced or moved back on the shaft. 
This action takes place without changing the cut-off valve travel or 
the relative movement of the two valves, since the throw of the two 
eccentrics is equal and constant. The Zeuner diagram for the 
Buckeye valve is worked out in a manner similar to that described 
for the Meyer valve. 

Two important claims are made for this valve gear : 
(1) On account of the valves moving in opposite directions at the 
instant cut-off occurs, cut-off is made very quickly, thus elim- 
inating quite largely wiredrawing and giving an indicator 
diagram having a sharp turn at the point of cut-off, resem- 



VALVE GEARS 85 

bling that given by a Corliss valve gear. This is illustrated by 

the diagram, Fig. 76. 
(2) On account of the constant travel of the valves, they wear better 

than those that control the regulation by varying the valve 

travel. 

This latter claim makes the gear particularly suited for the 
piston valve, since uneven wear or leakage is more liable to result 
from the packing rings if the valve movements are variable. 

DROP CUT=OFF GEARS 

The ordinary slide valve controls eight different events of the 
stroke, that is, admission, cut-off, release, and compression for both 
ends of the cylinder. A change in the setting of a plain slide valve 
that affects any one event on the crank end, let us say, will also change 
to a greater or less degree every other event of the stroke, on the 
head end as well as on the crank end; so that in setting a slide valve, 
the desired position for one event must usually be sacrificed in order 
to make the others less objectionable. 

In order to provide a better distribution of steam than is possi- 
ble with a single valve, some engines have four valves, two at each 
end of the cylinder. In horizontal engines, two valves are placed 
above the center line of the cylinder and two below, the upper being 
for admission and cut-off, the lower for release and compression. 
Since each valve controls but two events, a very satisfactory adjust- 
ment can be made and the extra complication and cost for large 
engines are more than overbalanced by the advantages gained, viz, 
a very much better distribution of steam; short steam passages and 
small clearances; separate ports for the admission of hot steam, and 
the exhaust of the same steam after expansion when its temperature 
has fallen; and finally the possibility of opening and closing the ports 
very rapidly, thus preventing wiredrawing. The small clearances, 
short ports, and separate admission and exhaust materially reduce 
the cylinder condensation, and thus effect a large saving in the steam 
consumption. 

When four valves are used for high speeds, the motions of all 
must be positive, that is, they must be connected directly to some 
mechanism that either pushes or pulls them through their entire 
motion, out for speeds up to 100 revolutions or so, a disengaging 



86 



VALVE GEARS 



mechanism may be used, and the valves may shut off themselves, 
either by virtue of their weight or by means of springs or dashpots. 




Fig. 77. Diagram of Reynolds-Corliss Drop Cut-Off Gear 

The valve is usually opened by means of links or rods moved by an 
eccentric and, at the proper point of cut-off, the rod is disengaged 
from the valve, which drops shut, hence the term "drop cut-off" gears. 
Reynolds=Corliss Gear. The most widely known drop cut-off 
gear is the Reynolds-Corliss, Figs. 77 and 78. It is often referred 
to as the Reynolds hook-releasing gear. An eccentric on the main 
shaft gives an oscillating motion to a circular disk, called the wrist 



dJ 



BELL C/?ANH- Z „ 




~~t-£ASHPQT BOD 



Fig. 7S. Details of Reynolds Hook 



plate, pivoted at the center of the cylinder. It transmits motion to 
each of the four valves through adjustable links known as steam rods 



VALVE GEARS 



87 



or exhaust rods, according to whether they move the admission or 
exhaust valves. 

The valves which are shown in section in Fig. 79 oscillate on 
cylindrical seats, and the position of the rods is so determined that 
they give a rapid motion to the valve when opening or closing, and 
hold it nearly stationary when either opened or closed. 

The Reynolds hook is shown in detail in Fig. 78. The steam 
arm is keyed to the valve spindle which passes loosely through a. 
bracket on which the bell-crank lever turns, and the spindle is packed 
to make a steam-tight joint where it enters the cylinder. Motion 
of the steam rod toward the right 
will turn the bell-crank lever and 
raise the hook stud. The hook 
(from which the gear derives its 
name) , pivoted on this stud, has at 
one end a hardened steel die with 
sharp, square edges, and at the 
other end, a small steel block with 
a rounded face. As the hook rises, 
the hook die engages the stud 
which is fastened to the steam 
arm, and one end of the steam 
arm is thus raised. This turn s the 
valve in its seat and admits steam. 
As the hook continues to rise, its 
stud moves in an arc above the Fi ^- 79 - Diag y a a ^: es s ^rsi«i I i eynolds " Corlis9 
valve spindle, and the round-faced 

block at its left-hand end strikes the knock-off cam, which causes 
the hook to turn about its stud and disengage the hook die from 
the stud die. In raising the steam arm, the dashpot rod is also raised 
and a partial vacuum is created in the dashpot. As soon, therefore, 
as the dies become disengaged, the dashpot quickly drops under the 
force of this vacuum, thus turning the steam arm and closing the 
valve. The striking of the left-hand end of the hook against the 
knock-off cam determines the point of cut-off' by releasing the valve 
at that instant. 

This cam is a part of the knock-off lever to which the governor 
cam rod is fastened. Any action of the governor which would cause 




88 VALVE GEARS 

the cam rod to move toward the right would cause this knock-of! 
lever to turn on its axis, the steam arm, and consequently lower the 
position of the knock-off cam. This would cause an earlier contact 
between the cam and the end of the hook, and consequently an earlier 
cut-off. By lengthening or shortening the governor cam rod, the 
point of cut-off can be adjusted to suit the engine load without 
changing the speed. 

There is a limit to this adjustment, for it can be shown that a 
Corliss gear operated by a single eccentric can not be arranged to 
cut off later than half-stroke. Suppose the eccentric is set just 90 
degrees ahead of the crank. Then it will reach its extreme position 
just as the piston gets to half-stroke. If, by that time, the hook which 
was rising and opening the admission valve has not yet struck the 
knock-off cam, it can not strike it at all, for any further motion will 
cause the hook to descend to its original position, that is, its position 
at the beginning of the stroke; the hook will not disengage from the 
steam arm stud at all and the bell crank will return, closing the valve 
in the same manner in which it opened it. Cut-off will then take 
place near the end of the stroke, but it will not be the sharp cut-off 
produced by the sudden drop when the dies are]disengaged. 

If the eccentric were set less than 90 degrees ahead of the crank, 
the cut-off could be arranged to occur later than half-stroke, but this 
is decidedly impracticable, for with such a position of the eccentric, 
the action of the valves at release and compression is spoiled. When 
it is necessary to cut off later than half -stroke, as sometimes happens 
on low-pressure cylinders of compound engines, it may be arranged 
for by means of two eccentrics, one set more than 90 degrees ahead 
of the crank to operate the exhaust valves, and one less than 90 
degrees ahead to operate the admission valves. 

The safety cam, Fig. 77, is an important part of a Corliss gear. 
If for any reason the engine governor should fail to act, due, for 
instance, to the breaking of its driving belt, the governor would drop 
to its lowest position, supply more steam to the engine, and allow 
it to run away. The safety cam prevents this by moving so far to 
the right that it strikes the hook when it descends to pick up the 
steam arm. The hook is consequently turned toward the right and 
then lifted without engaging the stud die; the valve consequently 
remains closed and the engine stops. 



VALVE GEARS 



89 



Nordberg Gear. The Nordberg type of drop gear is designed 
for high speed and hard service. Instead of having its steam arm 
supported on the valve spindle, it is supported on a bearing formed 
by an extension of the steam bonnet, and the arm is provided with 
two hook dies instead of one. To eliminate side strains these two 
dies are connected on 
either side of the dashpot 
rod. The release is ac- 
complished by means of 
an extension of the steam 
arm which rides in a 
slotted cam. The posi- 
tion of the latter is un- 
der control of the gov- 
ernor. The dashpot is 
also carried by the steam 
bonnet and is located 
above the gear. The 
spring type of dashpot is 
used to secure positive 
action at high speed. 

In Fig. 80, A A are 
the two parts of the steam 
arm to which the hook 
dies (not shown) are at- 
tached. B is the exten- 
sion of the steam arm, 
and it carries at its left 
end a roller which works 
in the releasing cam C. 
When this roller strikes 
the off-set, shown in the cam, it raises the arm B, which disengages 
the hook dies and allows the dashpot. rod D to snap the steam 
valve shut. E is the dashpot cylinder. F F are the driving rods, 
the one on the right being connected to the eccentric on the engine 
shaft, and the one on the left driving the valve gear for the other 
end of the cylinder. The rod G is connected indirectly to the gov- 
ernor and controls the point of cut-off by changing its position as 
the governor changes, according to the load on the engine. 




Fig. 80. Nordberg Drop Gear 

Courtesy of Nordberg Manufacturing Company, 

Milwaukee, Wisconsin 



90 



VALVE GEARS 



Brown Releasing Gear. In addition to the Reynolds hook, 
several other devices are in use for opening and releasing Corliss 
admission valves. Among them, the Brown releasing gear, Fig. 
81, may be noted. The steam rod and dashpot rod are arranged 
much the same as in the Reynolds gear. The governor cam rod 
operates a plate cam having a curved slot so shaped that it takes 
the place of both the knock-off and the safety cam of Fig. 78. The 
steam arm is keyed to the valve spindle and carries at its lower end 
a steel die which is free to slip up and down a small amount. The 
part of this gear corresponding to the Reynolds bell crank becomes 
a straight rocker pivoted at its middle; and the part corresponding 

to the Reynolds hook has at 
one end a die which engages 
the die of the steam arm, and 
at its other end a roller run- 
ning in the curved cam slot. 
This hook is really a bell- 
crank lever with arms that are 
not in the same place. The 
bearing on which it turns is 
carried on the lower end of 
the rocker and, therefore, is 
equivalent to a movable pivot 
similar to the hook stud of 
the Reynolds gear. 

In the position shown, the 
dies are engaged. Motion of 
the steam rod toward the right will move the lower end of the rocker 
toward the left, and consequently turn the valve spindle in a right- 
hand direction. This will open the valve and at the same time raise 
the dashpot rod. Meanwhile, the roller is moving toward the left 
in a circular part of the cam slot, the center of which is at the center 
of the valve spindle. This causes the steam arm and the bell-crank 
lever, which has the roller at one end, to move in such a way that 
there is no relative motion between them. As soon, however, as the 
roller comes to the point where it is forced to move out of this cir- 
cular path and move farther from the valve spindle, both arms of 
the bell-crank lever are turned downward, the dies become disen- 




Fig. 81. Brown Releasing Gear for Operating 
Luiliss Admission Valves 



VALVE GEARS 



91 



gaged, and the dashpot closes the valve. The slight up-and-down 
motion of the steam-arm die allows it to rise, while the hook die 
passes underneath when returning to re-engage for the next stroke. 
The makers claim that this gear permits a much higher speed than 
is possible with other Corliss gears. 

Greene ' Gear. Another well-known drop cut-off gear is the 
Greene, Fig. 82. The valves are of the gridiron type, sliding on 
horizontal seats, the admission valves parallel to the axis of the 
cylinder, and the exhaust valves at right angles to the axis of the 
cylinder and just below it. A A are rock shafts turning in fixed 
bearings. B B are the admission valve stems. C is a slide bar, 
receiving a reciprocating motion from an eccentric. T T are tappets 
connected to the slide bar. They 
move to and fro with the slide 
bar and can also move independ- 
ently up and down. They are 
made fast at their lower end to 
the gauge plate D, which slides 
through the guide E. The guide 
E is made fast to the governor 
rod F and through this means can 
be raised or lowered, thus reg- 
ulating the height of the tappets. 

As the slide bar moves toward the right, the right-hand tappet 
is brought into contact with the toe of the rocker, causing it to turn 
on its bearings and move the rock lever and the valve stem B toward 
the right, thus opening the admission valve. Since the tappet 
moves in a horizontal direction, while the toe of the rocker moves in 
an arc, it will be seen at once that they will soon become disengaged 
and release the valve, which is at once closed by a dashpot (not shown 
in the figure). If the governor raises the tappets, cut-off will be 
later. A nut at the bottom of the governor rod allows a proper 
adjustment of the guide and guage plate. As the slide bar C moves 
toward the right, the left-hand tappet comes in contact with the heel 
of the left-hand rocker and, both being beveled, the toe of the rocker 
rises in its socket, allowing the tappet to pass under. It then falls 
by its own weight and is ready to engage the tappet on its return and 
open the valve. In this gear, the disengagement of the valve throws 




qpzr 

Fig. 82. Greene Drop Cut-Off Gear 



92 



VALVE GEARS 



no load whatever on the governor, a distinct advantage over the 
Corliss gear. The action of the exhaust valves is not shown in the cut. 
Sulzer Gear. The Sulzer gear is a drop cut-off widely used in 
Europe. The valves are of the poppet type, lifting straight from 
conical seats, so that there is no friction. They are usually placed 
vertically above and below the cylinder axis and are operated by 
eccentrics from a shaft geared to the main shaft. The admission 

valves are lifted from their seats 
by suitable levers, then released 
by a device equivalent in action to 
the Reynolds hook and are quickly 
closed by the action of springs. 

The exhaust valves of all 
drop cut-off gears are compara- 
tively simple in their operation, 
and both in opening and closing 
are moved by the direct action of 
the exhaust rods, 

A common form of vacuum 
dashpot for closing admission 
valves is shown in Fig. 83. The 
rod coming down from the steam 
arm makes a ball-and-socket joint with the dashpot piston. The 
dashpot is often let down into the engine frame, as shown. When 
lifted, the piston produces a partial vacuum underneath it so that 
it tends to drop quickly as soon as the valve gear is released. On 
some of the largest modern engines where the valves are very heavy, 
steam-loaded dashpots are used; that is, the dashpot piston has steam 
pressure on one side, and an air cushion on the other prevents it from 
striking the bottom of the dashpot. 

CORLISS VALVE SETTING 

The setting of a Corliss valve gear is a much longer process 
than the setting of a plain slide valve, but is nevertheless a compara- 
tively simple matter, for the various adjustments are nearly all 
independent of one another. In gears like that shown in Fig. 77, 
the length of both the eccentric rod and the carrier rod are usually 
adjustable, and the former should be of such length that the carrier 




Fig. 83. Form of Vacuum Dashpot for 
Closing Admission Valves 



VALVE GEARS 



93 



Wfi/STPLATB 
STAND 




Fig. 84. 



Corliss Wrist Plate for Adjusting 
Carrier Rod 



arm swings equal distances on each side of a vertical line through 
its pivot, and the carrier rod should be adjusted until the wrist plate 
oscillates symmetrically about a vertical line through its pivot. 
Nearly all Corliss engines have one mark on the wrist plate hub and 
three on the* wrist plate stand, as 
shown in Fig. 84, and the wrist 
plate should swing so that A, the 
mark it carries, moves from C to 
D, but not beyond either one. 
When A is in line with B, the 
wrist plate is in mid-position. 
The valves are then not in their 
exact mid-position, but it is cus- 
tomary to regard them as being 
in mid-position, and to speak of 
the laps as the amount which 
the port is covered by the valve when the wrist plate is in mid-position. 
Adjusting Steam Lap. To set the valves, remove the bonnets 
or covers of the valve chambers on the side opposite the gear. The 
ends of the valves are circular, but on their inside the cross section 
is as shown in Fig. 85. On the 
end, in line with the finished 
edge of the valve and on the 
seat in line with the edge of 
the steam port, are marks, as 
shown in Fig. 85. When these 
marks coincide, the valve is 
either just opening or just 
closing, and when in any other 
position, the amount of open- 
ing or the amount by which 
the port is closed is shown 
directly by the distance between the marks. Block the wrist 
plate in mid-position, hook up the admission valves, and adjust 
the length of the steam rods by means of the right and left 
threads provided for the purpose, until the ports are covered by 
the amount of lap indicated in Table II, opposite the given size 
of engine. 




Fig. 85. 



Diagram Showing Method of Adjusting 
Steam Lap for Corliss Valves 



94 



VALVE GEARS 

TABLE II 
Standard Lap and Clearance Values 



Diameter of Cylinder 


Steam Lap 


Exhaust Clearance 


Inches 


Inches 


Inches 


12 


i 

4 


& 


14 to 16 


A 


h 


16 to 22 


3 

8 


& 


22 to 28 


£ 


& 


28 to 36 


1 

2 


I 


36 to 42 


1 


A 



Adjusting Exhaust Clearance and Lead. Next adjust the 
exhaust rods until the exhaust ports are open an amount equal to 
the clearance given in Table II. Set the engine on its head-end 
dead point, hook the carrier rod onto the wrist plate and in the direc- 
tion in which the engine is to run, turn the eccentric enough to open 
the head-end admission valve by a proper amount of lead; then the 
eccentric will be (90+0) degrees ahead of the crank. The proper 
amount of lead will depend upon both the design of the gear and 
the speed at which the engine is to run; and may vary from ^ inch 
for small engines to as much as ^ inch or ^ inch for large engines 
and those of higher speed. When the proper amount of lead has 
been obtained, fasten the eccentric on the shaft by means of the set 
screw and make sure by trial that the wrist plate moves to its extremes 
of travel. The dashpot rods must be adjusted so that when the 
dashpot piston is at its lowest position, the hooks, Fig. 78, descend 
just far enough for the hook dies to snap over the stud dies with 
about ^2 inch to rg inch to spare, depending on the size of the gear. 

Adjusting Cut=Off. To adjust and equalize the cut-off, lift the 
governor to about the position that it will occupy when running at 
normal speed, and put a block under the collar to hold it in this 
position. First, set the double lever at the right of the governor 
cam rods, so that it makes approximately equal angles with each rod, 
and then turn the engine over by hand until the piston has moved 
to the desired point of cut-off. Adjust the proper cam rod until 
the knock-off cam strikes the hook and allows the valve to close, 
then turn the engine to the point of cut-off on the other stroke and 
adjust the other cam rod in a similar manner. Now set the governor 



VALVE GEARS 95 

in the lowest position to which it could fall if there were no load 
on the engine, and set the safety cams so that in this position the 
hook can not open the valve. A latch is provided, on which the 
governor can be supported slightly above its lowest position, so 
that the valves can be opened by the hook when starting the 
engine. As soon as the engine speeds up, this latch must be 
moved aside, so that if the governor fails to act, it can drop to its 
lowest point, otherwise this latch would hold it just high enough 
so that the safety cams could not act. 

When Corliss gears are set, as here described, the position of 
the eccentric may not be quite right, due to an incorrect estimate 
of the amount of lead required. The error is likely to produce 
faulty release and compression as well as poor admission, but it can 
not be very serious, and the engine will turn over with its own 
steam, so that indicator diagrams may be taken. The final adjust- 
ments can then be determined from an examination of the diagrams. 

VALVE GEAR TROUBLES AND REMEDIES 
Importance of Keeping Valve Gear in Condition. The valve 
motion, or valve gear, is primarily responsible for the correct 
steam distribution in all steam engines. It follows, then, that in 
order to maintain efficient operating conditions the valve gear 
should receive constant careful attention in order that any irregu- 
larities which may develop can be detected at once and the fault 
corrected. A great many of the different gears used in American 
practice are described earlier in this text. In a number of cases 
the methods used in adjusting the gear and setting the valve have 
been given. For this reason the matter presented under this head- 
ing will be principally a discussion of the methods to be followed 
when trouble develops under conditions of service. 

Familiar Types. Of the many different types of valves gears 
described in the preceding pages, perhaps the most familiar ones 
are included under the following heads: 

(1) The direct-acting duplex pump valve gear 

(2) The plain D-valve or piston valve gears of the simple steam 

engine 

(3) The Corliss engine valve gear 

(4) The Stephenson link motion valve gear 

(5) The Walschaert radial valve gear 



96 VALVE GEARS 

DUPLEX PUMP VALVE GEAR 

Description. A great variety of valve gears are used in direct- 
acting steam pumps. The most common form, and in many 
respects the most reliable, is that illustrated in Fig. 50, in the 
text "Steam Engines". A pump such as shown in the illustra- 
tion is nothing more than two pumps combined. In this par- 
ticular design the motion of the piston rod of each pump is made 
use of in operating the valve of the other. In such a gear the 
only part which is made adjustable is the length of the valve rod. 
It is easily seen therefore that the setting of the valve is a com- 
paratively simple matter. 

Possible Troubles. After such a pump has been in service for 
some time, it may be necessary to dismantle it preparatory for 
removal to a machine shop for repairs. An accident may happen 
in which one of the operating arms, which are usually made of 
cast iron, becomes broken. In either case the operating engineer 
should be in a position to readjust the parts and properly set 
the valves after the necessary repairs have been made. 

Setting Valves. The valves of such a pump are usually of 
the D-type, but piston valves could be used to advantage if desired. 
In setting the valves the general procedure should be as follows: 

Preparation. Remove the steam chest or valve chest covers so 
that the movement of the valve relative to the ports can be measured. 

Measuring Valve Travel. Move each of the pistons as far as 
it will go against the head in one direction and make a pencil mark 
on the seat of each valve at its edge, the farthest from the center 
of its travel. Now move the pistons against the other cylinder 
heads and make pencil marks on the valve seats, but on the other 
edge of the valves. The marks on the valve seat indicate the 
travel of its valve, which should be symmetrical with the ports. 

Equalizing Valve Travel. If the travel is unequal, relative to 
the steam ports, the valve stem should be adjusted until the valve 
overtravels each steam port by the same amount. When the travel of 
each valve has been equalized, the setting may be considered finished 
and the valve chest covers may be replaced and parts connected. 

Variation in Conditions. In the adjustments just explained it 
is assumed that the gear was originally proportioned properly so 
as to cause the valve of each pump to open early enough to 



VALVE GEARS 97 

prevent the pistons from striking the cylinder heads. It some- 
times happens that the closing of the valves on the water end of 
the pump is such as to require a slightly different setting from 
that explained above in order that the pistons may be reversed to 
prevent striking. When such is the case the necessary adjust- 
ment should be made. Each individual case will probably need 
different treatment and cannot be anticipated. 

PLAIN SLIDE VALVE GEAR OF SIMPLE STEAM ENGINE 

Types. As previously explained the plain slide valve gear of 
the simple steam engine is the simplest of all steam engine valve 
gears. On account of its simplicity it is less liable to get out of 
adjustment or meet with an accident which would totally disable 
its action. The essential elements of this gear are shown in Figs. 3 
and 4, 6 to 10, 20 and 21. It is usually found constructed in 
one of three forms: (1) the form in which the valve receives 
its motion directly from the eccentric; (2) the form in which the 
valve receives its motion through the medium of a rocker arm in 
such a manner that the valve rod and eccentric rod move in the 
same direction; and (3) the form in which the valve receives its 
-motion through the medium of a rocker arm in such -a manner 
that the valve rod and eccentric rod move in opposite directions. 

Use of Rocker Arms. The use of the rockers mentioned in 
the forms (2) and (3) is usually made necessary by the design of the 
engine, which is such that the valve stem and eccentric cannot 
be placed so they will be in the same straight line. In the-'adjust- 
ment of such gears it is essential that the rocker be so located 
that it will vibrate equally on either side of a vertical line drawn 
through the fulcrum point. If the rocker is not adjusted as 
directed, the valve will receive a motion which may be faster in 
one direction than the other even though its travel is equalized. 
When such conditions exist it- is impossible to secure a valve 
setting which will give a correct distribution. 

Slipped Eccentric. A trouble which is sometimes experienced 
when the engine is in operation is caused by the eccentric becom- 
ing loose on the shaft and slipping around in such a way as to 
reduce the power of the engine or perhaps cause the engine to 
stall. This usually happens in engines where the eccentric is held 



98 VALVE GEARS 

in position by means of a set screw. If a key is used the trouble 
is seldom experienced. 

Method of Correction. When it does happen that the eccen- 
tric slips, the operating engineer can get a setting which, while 
not absolutely correct, will permit the engine to be operated with 
but little loss of time by following the directions here given: 

First, set the engine on the head or crank end dead center, 
by inspection, with as much accuracy as is possible under the cir- 
cumstances. Second, turn the eccentric around the shaft in the 
direction the engine is to run until steam will just begin to blow 
out at the cylinder drain cock on the end in question when the 
throttle is opened slightly. When this position is found, tighten 
the set screw in the eccentric temporarily. Third, turn the engine 
over to the other dead center and see if steam blows from the cor- 
responding cylinder drain cock with the same degree of freedom. 
If it does the eccentric may. be said to be in the correct position 
and the set screw may be securely tightened. When this is done 
the engine will be ready to again assume its duties. At the first 
opportunity the valve setting should be carefully checked by one 
of the methods described earlier in this text. 

Increasing Power Capacity. It frequently happens in small 
plants using a plain slide valve engine that additional machines 
will be added from time to time until the engine finally becomes 
overloaded under ordinary conditions of operation. Under such 
circumstances the operator is asked to devise means of increas- 
ing the power delivered by the engine. This can be accomplished 
in one of the following ways: (1) by increasing the speed of the 
engine; (2) by increasing the pressure carried by the boiler; (3) by 
increasing the point of cut-off; and (4) by the combination of any 
two or all of the above methods. 

Importance of Boiler Capacity. An examination into the methods 
given above reveals the fact that in every case additional load will be 
placed on the boiler. If the boiler capacity is sufficient to carry the 
additional load, then the problem can be solved, otherwise it cannot. 

Increasing Speed. If the power is increased by increasing the 
speed of the engine to any very great degree, it will be necessary 
to change the size of the belt pulleys on the engine and line shaft 
in order not to disturb the speed of the machines. 



VALVE GEARS 99 

Increasing Boiler Pressure., In increasing the power by 
increasing the boiler pressure, no changes are necessary unless it 
is thought advisable to replace any or all of the high-pressure 
steam pipe and fittings with extra heavy grade. 

Lengthening Point of Cut-Off. If it is desired to increase the 
power by lengthening the point of cut-off, this [can be accom- 
plished by removing the valve and planing off the ends, thus reduc- 
ing the steam lap the desired amount to give the increased cut-off. 
It is very essential to remove the same amount from each end 
of the valve, otherwise the steam lap would be different for each 
end. If the engine was originally cutting off at one-half stroke 
and it is desired to have the cut-off increased to three-fourths 
stroke, the amount of metal which should be removed to give the 
desired condition can easily be determined by drawing a Zeuner 
diagram from the valve in question. When the valve is finally 
reconstructed and placed in position in the steam or valve chest, 
it will be necessary to change the angle of advance of the eccen- 
tric in order to secure the proper amount of lead. To secure the 
proper setting it would be advisable to follow the directions given 
earlier in this text. 

Use of Double Valve. As has been previously pointed out, 
the plain D-valve possesses certain objectionable features in the 
matter of steam distribution which is partially overcome by the 
use of a double valve. The Meyer valve is perhaps the most 
common form of double valve, a description of which is given on 
pages 73 to 78 of this text. 

Setting Meyer Valve. In setting the Meyer valve, the main 
valve is set in the same manner as the ordinary simple D-valve. 
This main valve controls the admission, release, and compression 
points, while the riding, or secondary, valve controls the point of 
cut-off. Having correctly set the main valve, connect the riding 
valve to its eccentric and adjust the rods so that its travel is 
equal on each side of its central position, in exactly the same way 
as directed for the simple D-valve. When this is done, place the 
piston at the point where cut-off is desired and rotate the riding 
eccentric in the direction the engine is to run until a point is 
reached where the valve is just cutting off. When this point 
is reached fasten the riding eccentric to the shaft. Next place the 



100 VALVE GEARS 

piston at the same relative position on the other stroke, and, if 
cut-off is just occurring, the valve may be said to be correctly 
set and the riding eccentric securely fastened. If cut-off does not 
occur at the same point on each end, make adjustments of the 
eccentric and valve rod until the cut-off points are equalized. 

Pounding or Knocking. The question of pounding or knock- 
ing is discussed in "Steam Engines", but since this is frequently 
caused by improper valve setting, it seems well to give this 
troublesome matter a brief consideration. 

Indications of Faulty Valve Action. If an annoying pound 
is heard which is difficult to locate, it is probably due to an 
improperly set valve. If this is the real cause of the trouble, it 
will be easily shown by indicator cards taken from the engine 
when under regular operating conditions. If the pound is due to 
valve action it will be revealed in the indicator cards in one or all 
of the following three things: (1) by compression beginning so 
early that the compression pressure exceeds the steam line pres- 
sure, thus causing the valve to be raised from its seat until the 
admission point is reached when the valve is forced to its seat with 
a "slam"; (2) by admission occurring so late that the lost motion 
first "runs out" and is then taken up after steam has been admitted; 
and (3) by the unequal distribution of power between the two ends of 
the cylinder, thus causing nearly all the work to be done in one end. 

Correction of Fault. By following the directions as previously 
given for valve setting by measurement or by indicator, it becomes 
a comparatively small matter to correct the trouble. 

CORLISS VALVE GEAR 

Description. The Corliss valve gear is the most widely 
known of all the types of so-called "drop cut-off" valve gears. It 
is more economical than most other types from the standpoint of 
steam consumption but, on account of its peculiar construction and 
multiplicity of parts, is not adapted for high-speed work, say, above 
100 revolutions per minute. Directions for setting a Corliss valve 
gear have been presented earlier in this text and need not be repeated 
here, but there is a word of caution which should be emphasized. 

Possible Troubles. The rods connecting the steam valve 
arms with the dash pots should be adjusted so that when down 



VALVE GEARS 101 

as far as they will go and with the wrist plate in its extremes 
of travel the stud die on the valve arm will just clear the shoulder 
on the hook die. If the rod is left too long, the steam valve 
stem will probably be bent, the valve arm broken, or the dash 
pot rod bent or broken. It may happen that the jar from the 
action of the dash pot will cause the dash pot rod to become 
loosened while in service. If this occurs the parts just mentioned 
may be broken in a manner similar to that when the rod is left 
too long in setting. Again, if the dash pot rod is left too short, 
the hook will not engage and, consequently, the valve will not open. 

STEPHENSON VALVE GEAR 

Extent of Use. The Stephenson gear, or link motion, as it is 
commonly called, is one of the oldest and best known types of 
reversing gears in use in the United States. For a great many 
years it was used almost to the exclusion of all other types of 
gears on American locomotives. Of recent years, however, its use 
has declined until today we find only comparatively few American 
locomotives equipped with the Stephenson reversing gear. The 
use of this gear is not confined entirely to locomotive service. In 
fact, it is made use of on steam engines in many classes of service, 
such as, steam tractors, steam road rollers, stationary engines, and 
hoisting engines. 

Characteristics. Increase of Lead in Open-Rod Construction. 
One of the characteristics of the Stephenson reversing gear is that 
the lead of 'the valve increases from full to mid gear for open-rod 
construction and decreases from full to mid gear for crossed- 
rod construction. The crossed-rod construction is seldom used on 
engines unless service conditions are such as to make necessary 
its manipulation by the use of the reversing lever. The feature of 
increasing lead from full to mid gear, under certain conditions, 
is desirable on locomotives used for passenger service. In such 
instances the engineer will usually start the train with the 
reverse lever at or near the full gear position where the lead is a 
minimum and as the speed increases will bring the reverse lever 
nearer and nearer the central position where the lead is greater. 
This feature considered by itself is desirable since for best working 
conditions the lead should increase with the speed. 



102 VALVE GEARS 

Back- Up Eccentric. One very desirable feature of the Stephen- 
son gear is that it may be set to secure almost any steam dis- 
tribution desirable. This is accomplished by making use of the 
"back-up" eccentric. Applying this method to setting the valves 
will, of course, disarrange the reverse, or "back-up", conditions but 
the "go-ahead" conditions can be almost perfectly secured. 

Possible Troubles. Lost Motion in Driving Boxes. In the 
use of Stephenson gears on locomotives there is one condition 
which frequently arises but is rarely considered. The condition 
referred to is the development of lost motion in the driving boxes. 
In such cases, the eccentric being attached to the axle, the full 
amount of this lost motion is delivered to the valve with the link 
working in full gear. In certain other types of gears this condition 
would produce but very little change in the movement of the valves. 

Effect of Vertical Motion of Engine. Another condition which 
affects the steam distribution when a Stephenson gear is used is 
the vertical motion of the engine on its springs caused by irregu- 
larities in the track. 

Setting Valve. In setting the valve on an engine using the 
Stephenson gear, the fundamental principles involved are exactly 
the same as those given for the setting of a plain slide valve gear. ■ 
We need to keep constantly in mind, however, that there are two 
eccentrics and two eccentric rods to deal with instead of one. 

Typical Plain Slide Valve Setting. As an example let us 
consider the case of an engine fitted with a plain slide valve gear. 
Suppose it is desired to give the valve a lead of -^ inch on both 
the head and crank ends. An examination of the valve discloses 
the fact that the lead on the head end is § inch and that on the 
crank end is -^ inch, which is the desired amount. The problem 
is to reduce the lead on the head end A inch without disturbing 
the lead on the crank end. This problem can be solved by reduc- 
ing the lead on the head end -£± inch by changing the length of 
the valve rod and an additional -£± inch by changing the angle 
of advance of the eccentric on the shaft. If the work is carefully 
done the results should show a lead of ^ inch on both ends, unless 
the angularity of the eccentric rod is a very considerable amount. 

Stephenson Valve Setting. Now suppose that the simple slide 
valve gear on this engine has been replaced by a Stephenson revers- 



VALVE GEARS 103 

ing gear and that an examination of the valve with the reverse 
lever in full gear position shows the lead on the head end to be 
J inch and that on the crank end -^ inch for the forward position 
of the reverse lever, while for the backward position the leads on 
both the head and crank ends are found to be correct, namely, 
A inch. In this case, the same as before, it is desired to secure a 
lead of A inch on each end when the ieverse lever is in both the 
forward and backward positions. To accomplish this with the 
reverse lever in the full forward position, it will be necessary to 
reduce the lead -f^ inch by changing the length of the eccentric 
rod and an additional A inch by changing the position of the 
eccentric on the shaft. If the work is carefully done the desired 
results will be approximately secured. 

Differences in the Two Settings. In the example just presented 
it should be noted that in the case of the simple gear the adjust- 
ments were made on the valve rod and eccentric, while in the case 
of the Stephenson gear they were made on the eccentric rod and 
eccentric. This correction of one-half the error on the eccentric 
and one-half on the eccentric rod, instead of on the valve rod, is 
necessary in order to permit the conditions on the reverse direction 
to remain unchanged. Other adjustments of a like nature can be 
made in a similar manner. 

Variation in Conditions. Unfortunately, in practice, the 
Stephenson reversing gears are not always constructed so as to 
permit all the adjustments mentioned above. In such instances 
a compromise will have to be made. 

WALSCHAERT GEAR 

Extent of Use. The Walschaert gear has been used abroad 
for many years but never attained prominence in this country 
until ten or twelve years ago. It represents the most satisfactory 
type of radial reversing gear now in service in the United States. 
It is now being equipped on approximately 80 per cent of all new 
American locomotives, the remaining 20 per cent being fitted with 
the Stephenson gear. Its use, however, is confined almost exclu- 
sively to locomotive service. 

Comparison with Stephenson Gear. One of the chief advan- 
tages of the Walschaert gear is the accessibility of all the parts and 



104 VALVE GEARS 

the comparative ease with which repairs can be made. The parts 
of the gear being located outside, the space below the boiler may be 
used for other parts not so necessarily accessible. The chief point 
in which the Walschaert gear differs from the Stephenson gear on 
the action of the valve is that the former gives a constant lead for 
all positions of the reverse lever. Both gears are adaptable for 
use with any form of locomotive valve yet designed. The usual 
construction of the Walschaert gear is such as to permit little or 
no adjustments being made on the road. It is unusually free from 
any inclination of the parts to cause trouble through heating. Cases 
are known where improperly designed gears gave some trouble by 
the eccentric rod pins heating due to the twisting effect between the 
driving wheels and engine frame caused by unusual conditions of 
track and service. This, however, is a matter easily corrected. 

Lost motion in the driving boxes produces much less effect 
on the motion of the valve when a Walschaert gear is used than 
when a Stephenson gear is employed. Neither does the up-and- 
down motion of the engine on its springs affect the steam dis- 
tribution unless the connection of the eccentric rod to the link 
foot is placed at too high a point above the center line of the 
axle. In all well-designed Walschaert gears it is necessary that 
the trunnion upon which the link oscillates be fixed at an unvary- 
ing distance from the cylinder. In the fulfillment of this require- 
ment it will be observed that the link bracket is invariably 
attached to the guide bearer, or yoke, and the slide for the valve 
stem is mounted on the upper guide bar. In some types of loco- 
motives the construction is such that a large cast-steel bracket is 
laid across, joining the bars of the engine frame on both sides just 
back of the guide yoke, which acts as a frame binder and brace 
and a carrier for the link bracket. In still another type, the large 
casting is bolted to the guide yoke as well as the frame, thus 
forming a most substantial construction. 

Repairs. With the Walschaert gear in service, if a break 
occurs within the valve gear, the difference in time consumed in 
making the temporary repairs necessary to get the engine moving 
under its own steam is greatly in its favor. This is one of the 
principal reasons for its adoption, since it means less time lost 
in del a vs. 



INDEX 



INDEX 

A PART PAGE 

American Thompson indicator I, 8 

Assembling and adjusting indicator I, 30 

adjustment I, 32 

card and pencil I, 32 

length of indicator card I, 32 

assembling Crosby indicator I, 30 

testing action I, 31 

B 

Brown releasing gear II, 90 

Brumbo pulley I, 21 

C 

Corliss valve setting II, 92 

cut-off, adjusting II, 94 

exhaust clearance and lead, adjusting II, 94 

steam lap, adjusting II, 93 

Crosby device I, 19 

Crosby indicator I, 3 

Crosby indicator, assembling of I, 30 

attaching spring I, 31 

connecting piston rod I, 30 

Crosby reducing wheel I, 26 

D 

Double valve gears II, 73 

Meyer II, 73 

shifting eccentric II, 78 

Thompson automatic II, 81 

Drop cut-off gears II, 85 

Brown releasing II, 90 

Greene II, 91 

Nordberg gear II, 89 

Reynolds-Corliss II, 86 

Sulzer ,. . . II, 92 

G 

Gooch link II, 63 

Greene gear II, 91 

H 

Hackworth gear f . . II, 64 



INDEX 



I 



Indicator cards, interpretation of 

cards showing valve troubles 

early compression 

early cut-off 

excessive back pressure 

late admission 

wire drawing 

gas engine cards 

steam cards showing miscellaneous troubles. 

faulty valve arrangement 

long indicator cord 

lost motion 

speed governing . . , 

sticky indicator piston 

tight indicator piston 

valve trouble 

variable cut-off 

theoretical diagram 

Indicator spring testing 

apparatus 

continuous diagrams 

detent attachment 

engine connection 

reducing motions. . 

Brumbo pulley 

Crosby reducing wheel 

pantograph 

reducing wheel 

simultaneous indicator cards 

spring calibration 

Indicator troubles and remedies 

adjustment of guide pulley 

adjustment of pencil pressure 

attachment of indicator 

drum spring tension 

miscellaneous precautions 

care in handling indicator 

causes of incorrect indication 

importance of rules 

lubrication 

modifications for high speeds 

necessity for care in using indicator 

reducing motions 



Joy gear II, 



G7 



INDEX 3 

M PART PAGE 

Marshall gear II, 66 

Meyer valve II, 73 

P 

Physical theory I, 41 

heat I, 43 

temperature I, 43 

thermometers I, 43 

unit of heat quantity I, 44 

horsepower I, 44 

brake .....:.. I, 48 

indicated I, . 45 

mechanical efficiency I, 48 

piston displacement I, 48 

pressure ... a I, 42 

absolute : . . . . I, 42 

atmospheric I, 42 

boiler I, 42 

work I, 42 

Planimeter, instructions for use of I, 40 

R 

Reynolds-Corliss gear II, 86 

S 

Separating calorimeter : . . . . I, 60 

Simple engine, reversing of II, 37 

definitions II, 38 

direct valve 

engine running over II, 39 

engine running under II, 39 

indirect valve 

comparisons and comments II, 40 

engine running over II, 39 

engine running under II, 40 

Slide valve, design of ■. II, 31 

area of steam port II, 31 

bridge, width of II, 34 

exhaust port, width of II, 34 

lead II, 35 

point of cut-off II, 34 

reversing simple engine II, 37 

width of steam port II, 33 

Slide valve, modifications of II, 46 

balancing steam pressure II, 46 

application of various types II, 49 

double-ported valve II, 47 



4 INDEX 

Slide valve, modifications of (continued) part page 
balancing steam pressure 

piston valve II, 46 

trick valve II, 48 

reversing mechanism II, 50 

by means of one eccentric II, 50 

by means of two eccentrics and curved or straight links . . II, 52 

by means of two eccentrics and gab-hooks II, 51 

Steam 

kinds of I, 51 

saturated or dry I, 51 

superheated I, 54 

wet I, 51 

properties of I, 48 

calorimetric measurements I, 57 

feed water temperature I, 56 

saturated vapor I, 48 

tables I, 50 

thermal efficiency I, 63 

volume and weight of I, 61 

Steam engine indicators I, 1-89 

assembling and adjusting indicator I, 30 

indicator cards, interpretation of I, 64 

indicator spring testing I, 11 

physical theory I, 41 

planimeter I, 40 

steam, properties of, I, 48 

taking cards , I, 33 

testing steam engines I, 75 

troubles and remedies I, 84 

types I, 2 

Steam engines, testing of I, 75 

calorimeters I, 78 

factors considered I, 76 

gauges I, 78 

indicators I, 77 

meters I, 77 

Prony brakes I, 78 

modern band type I, 81 

original type I, 78 

rope type > I, 80 

scales I, 77 

speed counter I, 83 

thermometers I, 77 

Stephenson link motion II, 53 

application to expansion and cut-off II, 57 

location of link block . . . r II, 56 

relative position of eccentric rods. . II, 54 

valve ellipse diagram II, 60 



INDEX 5 

Stephenson link motion (continued) part page 

Zeuner diagram for Stephenson gear II, 58 

Sulzer gear II, 92 



Tables 

constants of indicator springs 

effect of changing lap, travel, and angular advance I 

engine constants 

properties of saturated steam 

standard lap and clearance values I 

Tabor indicator 

Taking cards 

condition of indicator 

indicator card analysis 

determination of mean effective pressure by planimeter . . 

events of cycle 

meaning of lines of diagram 

measurement of clearance 

pressures 

sample indicator card 

Thompson automatic valve gear I 

Throttling calorimeter 

Troubles and remedies 

indicator 

valve gear I 

Types of steam engine indicators 

American Thompson indicator 

Crosby indicator 

Tabor indicator 

Watt indicator ? 



30 
47 
52, 53 
94 
6 
33 
33 
34 
40 
37 
34 
36 
38 
34 
81 
58 

84 
95 
2 
8 
3 
6 
2 



Valve characteristics II, 1 

eccentric II, 2 

function II, 1 

lead II, 9 

effect of II, 10 

valve motion II, 3 

analysis of II, 6 

effect of change of lap II, 8 

valve with lap II, 5 

Valve diagrams II, 17 

effect of changing lap, travel, or angular advance II, 29 

Zeuner II, 17 

Valve gear, radial type of II, 64 

Hackworth b II, 64 

Joy , II, 67 



6 INDEX 

Valve gear, radial type of (continued) part page 

Marshall II, 66 

Walschaert II, 67 

Valve gears , II, 1-104 

characteristics II, l 

Corliss valve setting II, 92 

double valve gears II, 73 

drop cut-off gears . II, 85 

radial type of . II, 64 

shifting link type of II, 53 

slide valve 

design of II, 31 

modifications of II, 46 

troubles and remedies II, 95 

comparison of Walschaert and Stephenson gears. II, 103 

Corliss valve gear II, 100 

duplex pump valve gear II, 96 

importance of keeping valve gear in condition II, 95 

increasing power capacity II, 98 

plain slide valve gear II, 97 

pounding or knocking it, 100 

setting valves II, 96, 99, 102 

slipped eccentric II, 97 

Stephenson valve gear II, 101 

use of double valve II, 99 

Walschaert gear .' II, 103 

valve diagrams II, 17 

valve setting II, 41 

Valve setting II, 41 

possible adjustments II, 41 

to put engine on center II, 41 

to set valve for equal cut-off II, 44 

to set valve for equal lead II, 43 

Valve terms, analytical summary of II, 11 

angle of advance II, 12 

compensation of cut-off II, 14 

displacement II, 11, 13 

eccentricity II, 11 

inequality of steam distribution II, 12 

lap II, 11 

lead II, 12 

mid-position II, 11 

rocker II, 16 

valve travel II, 11 

W 

Walschaert gear II, 67 

adjustment of II, 70 

analvsis of valve motion : II, 68 



INDEX 7 

Walschaert gear (continued) part page 

dimensions of parts II, 73 

link motion II, 70 

Zeuner diagram for II, 71 

Watt indicator I, 2 

Z 

Zeuner diagram for Stephenson gear II, 58 

Zeuner diagram for Walschaert gear II, 71 

Zeuner diagrams II, 17 

properties of II, 21 

study of valve motion from diagram II, 19 



